Add api_identifier as an accepted configurable

[QWYNG]

Changes

I'd like to fetch a api token with audience that my api identifier.
When I use auth0 with custom domain, I need to specify an audience that is different than the domain.
But I can not unable to do this now.

The :audience parameter is synonymous with the OIDC ID Token and is also used to generate the login URL, so it's confusing.
Auth0's Client Credentials Flow asks the audience for an API identifier.

:custome_domein and :sub were suggested at #189 , but I thought there was a more direct way to express it.
so I think the parameter :api_identifier is not confusing, What are your thoughts?

References

This issue has been discussed at #189 as well.

Testing

Please describe how this can be tested by reviewers. Be specific about anything not tested and reasons why. If this library has unit and/or integration testing, tests should be added for new functionality and existing tests should complete without errors.

• This change adds unit test coverage
• This change adds integration test coverage
• This change has been tested on the latest version of Ruby

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors
• Rubocop passes on all added/modified files
• All active GitHub checks have passed

[QWYNG]

@davidpatrick thank you!

[nigelwtf]

Super late to the party on this one - but thought it better to comment than to raise a new issue.

Using a different audience & domain is probably a really common requirement - pretty much anyone using this library and who has a custom domain, but I only figured out this existed by reading through unit tests (I even tried Googling it afterwards just to make sure 😂).

Would it be worthwhile documenting api_identifier as a way to specify the audience? I'd be happy to raise a PR if someone steers me in the right direction.