feat: Custom Token Exchange support

[rmad17]

Changes

Please describe both what is changing and why this is important. Include:

• Endpoints added, deleted, deprecated, or changed
• Classes and methods added, deleted, deprecated, or changed
• Screenshots of new or changed UI, if applicable
• A summary of usage if this is a new feature or change to a public API (this should also be added to relevant documentation once released)
• Any alternative designs or approaches considered

References

Please include relevant links supporting this change such as a:

• support ticket
• community post
• StackOverflow post
• support forum thread

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 the platform/language or why not

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