NNG v2.0 Migration

[shikokuchuo]

This branch contains the updates to support the NNG v2.0 API.

CI will currently fail as I have not yet updated the bundled versions of mbedtls + NNG.

• Works with system installations of mbedtls 3.6.5 and NNG v2.0 alpha 6 in /usr/local/lib

Internal changes:

• New interfaces: nng_close() -> nng_socket_close(), nng_recv_aio() -> nng_socket_recv(), nng_send_aio() -> nng_socket_send()
• New library initialization nng_init(NULL) is called from package load routine. This allows nng_fini() to be safely called on package unload (previously avoided)
• Entirely updated http semantics with a http_conn struct instead of request/responses
• New udp:// transport

Revised behaviour:

• opt() / opt<-() now no longer support string / uint64 / NULL option types
• parse_url() now returns a character vector of length 7 rather than 10
• pipe_notify() must be called before the socket listen / dial events
• Some error values have been internally updated

Tests have been updated for above behavioural changes and test clean

• Completely clean local Valgrind runs

TODOs:

• For opt(), return options on the stream dialer / listener rather than the stream
• Investigate remaining http TLS test failures (failure to return errorValues when using incorrect client credentials)
• Update bundled mbedtls to 3.6.5 (+ configure support for DTLS etc.)
• Update bundled NNG v2 (beta)
• Update docs for changed behaviour

Support for new features will be implemented in separate PRs that target this one.

FYI @lionel-