feat: support custom idp login when recording test fixtures

[kuntzed]

Purpose

• Add support for setting the identity provider from environment variable BTP_IDP during provider configuration. This is implemented analogously to BTP_USERNAME and BTP_PASSWORD including the required redaction of recorded fixtures. To prevent impact on existing fixtures, identity provider values are replaced by the empty string during redaction.

Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[x] Feature
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

• Test the code

1. go test ./...
2. Try local recording of text fixtures and use a custom idp user by setting all three of the env vars BTP_USERNAME, BTP_PASSWORD, BTP_IDP, setting the latter to the ias tenant 'terraformint'

What to Check

Verify that the following are valid

1. The tests should still be successful
2. The resulting fixtures should not contain any information about the selected idp (ias tenant)

Other Information

Also fixes a warning when the username has an unknown value during provider configuration.

Resolves #243

[kuntzed]

We should discuss if/when it would make sense to adapt the request redaction to replace any idp with some test value instead of the empty string, although this would mean that all fixtures need to be re-recorded. While at it, we could also replace the issuer returned during login to a test value to get rid of the hardcoded values containing 'accounts.sap.com' and 'accounts400.ondemand.com'.