libdns/rfc2136 added as a provider

[reind33r]

Hi!

I added the RFC2136 provider from libdns, following the pattern from the other already-integrated providers.

I figure it can make sense for others to have it, if they host their emails they might host their DNS too.

It builds and the tests pass, I believe, I have no experience so I just executed the commands from the CI/CD workflow and nothing failed.

/src # go test ./... -coverprofile=coverage.out -covermode=atomic
?       github.com/foxcpp/maddy [no test files]
?       github.com/foxcpp/maddy/cmd/maddy       [no test files]
?       github.com/foxcpp/maddy/cmd/maddy-pam-helper    [no test files]
?       github.com/foxcpp/maddy/cmd/maddy-shadow-helper [no test files]
ok      github.com/foxcpp/maddy/framework/address       0.003s  coverage: 73.6% of statements
?       github.com/foxcpp/maddy/framework/buffer        [no test files]
ok      github.com/foxcpp/maddy/framework/cfgparser     0.003s  coverage: 91.7% of statements
ok      github.com/foxcpp/maddy/framework/config        0.003s  coverage: 42.9% of statements
ok      github.com/foxcpp/maddy/framework/config/lexer  0.001s  coverage: 88.5% of statements
?       github.com/foxcpp/maddy/framework/config/module [no test files]
?       github.com/foxcpp/maddy/framework/config/tls    [no test files]
ok      github.com/foxcpp/maddy/framework/dns   1.006s  coverage: 25.2% of statements
?       github.com/foxcpp/maddy/framework/exterrors     [no test files]
ok      github.com/foxcpp/maddy/framework/future        1.005s  coverage: 75.8% of statements
?       github.com/foxcpp/maddy/framework/hooks [no test files]
?       github.com/foxcpp/maddy/framework/log   [no test files]
ok      github.com/foxcpp/maddy/framework/logparser     0.001s  coverage: 87.5% of statements
?       github.com/foxcpp/maddy/framework/module        [no test files]
ok      github.com/foxcpp/maddy/internal/auth   0.004s  coverage: 51.3% of statements
?       github.com/foxcpp/maddy/internal/auth/dovecot_sasl      [no test files]
?       github.com/foxcpp/maddy/internal/auth/external  [no test files]
?       github.com/foxcpp/maddy/internal/auth/ldap      [no test files]
?       github.com/foxcpp/maddy/internal/auth/netauth   [no test files]
?       github.com/foxcpp/maddy/internal/auth/pam       [no test files]
ok      github.com/foxcpp/maddy/internal/auth/pass_table        0.094s  coverage: 30.7% of statements
ok      github.com/foxcpp/maddy/internal/auth/plain_separate    0.011s  coverage: 28.3% of statements
?       github.com/foxcpp/maddy/internal/auth/shadow    [no test files]
?       github.com/foxcpp/maddy/internal/authz  [no test files]
?       github.com/foxcpp/maddy/internal/check  [no test files]
?       github.com/foxcpp/maddy/internal/check/authorize_sender [no test files]
?       github.com/foxcpp/maddy/internal/check/command  [no test files]
ok      github.com/foxcpp/maddy/internal/check/dkim     0.004s  coverage: 89.3% of statements
ok      github.com/foxcpp/maddy/internal/check/dns      0.009s  coverage: 81.6% of statements
ok      github.com/foxcpp/maddy/internal/check/dnsbl    0.003s  coverage: 45.7% of statements
ok      github.com/foxcpp/maddy/internal/check/milter   0.003s  coverage: 7.3% of statements
?       github.com/foxcpp/maddy/internal/check/requiretls       [no test files]
?       github.com/foxcpp/maddy/internal/check/rspamd   [no test files]
?       github.com/foxcpp/maddy/internal/check/spf      [no test files]
?       github.com/foxcpp/maddy/internal/cli    [no test files]
?       github.com/foxcpp/maddy/internal/cli/clitools   [no test files]
?       github.com/foxcpp/maddy/internal/cli/ctl        [no test files]
ok      github.com/foxcpp/maddy/internal/dmarc  0.007s  coverage: 91.1% of statements
?       github.com/foxcpp/maddy/internal/dsn    [no test files]
?       github.com/foxcpp/maddy/internal/endpoint/dovecot_sasld [no test files]
?       github.com/foxcpp/maddy/internal/endpoint/imap  [no test files]
?       github.com/foxcpp/maddy/internal/endpoint/openmetrics   [no test files]
ok      github.com/foxcpp/maddy/internal/endpoint/smtp  1.775s  coverage: 70.6% of statements
?       github.com/foxcpp/maddy/internal/imap_filter    [no test files]
?       github.com/foxcpp/maddy/internal/imap_filter/command    [no test files]
?       github.com/foxcpp/maddy/internal/libdns [no test files]
?       github.com/foxcpp/maddy/internal/limits [no test files]
?       github.com/foxcpp/maddy/internal/limits/limiters        [no test files]
ok      github.com/foxcpp/maddy/internal/modify 0.003s  coverage: 37.0% of statements
ok      github.com/foxcpp/maddy/internal/modify/dkim    0.776s  coverage: 70.6% of statements
ok      github.com/foxcpp/maddy/internal/msgpipeline    0.007s  coverage: 74.2% of statements
?       github.com/foxcpp/maddy/internal/proxy_protocol [no test files]
ok      github.com/foxcpp/maddy/internal/smtpconn       0.008s  coverage: 35.3% of statements
?       github.com/foxcpp/maddy/internal/smtpconn/pool  [no test files]
?       github.com/foxcpp/maddy/internal/storage/blob   [no test files]
ok      github.com/foxcpp/maddy/internal/storage/blob/fs        0.088s  coverage: 40.0% of statements
ok      github.com/foxcpp/maddy/internal/storage/blob/s3        0.113s  coverage: 65.7% of statements
ok      github.com/foxcpp/maddy/internal/storage/imapsql        0.002s  coverage: 0.8% of statements [no tests to run]
ok      github.com/foxcpp/maddy/internal/table  1.577s  coverage: 32.2% of statements
?       github.com/foxcpp/maddy/internal/target [no test files]
ok      github.com/foxcpp/maddy/internal/target/queue   1.506s  coverage: 75.4% of statements
ok      github.com/foxcpp/maddy/internal/target/remote  0.049s  coverage: 65.1% of statements
ok      github.com/foxcpp/maddy/internal/target/smtp    0.011s  coverage: 82.4% of statements
?       github.com/foxcpp/maddy/internal/testutils      [no test files]
?       github.com/foxcpp/maddy/internal/tls    [no test files]
?       github.com/foxcpp/maddy/internal/tls/acme       [no test files]
?       github.com/foxcpp/maddy/internal/updatepipe     [no test files]
?       github.com/foxcpp/maddy/internal/updatepipe/pubsub      [no test files]
?       github.com/foxcpp/maddy/tests   [no test files]
/src # cd tests/
/src/tests # ./run.sh 
warning: no packages being tested depend on matches for pattern github.com/foxcpp/maddy/pkg/...
PASS
ok      github.com/foxcpp/maddy/tests   9.672s

[foxcpp]

Some minor edits are needed, LGTM otherwise.