Mark WithClientAssertion API as experimental

[gladjohn]

Fixes #- Mark API as experimental

Changes proposed in this request
This pull request refactors how client assertion delegates are registered in ConfidentialClientApplicationBuilder and updates related unit tests to enable experimental features required for these APIs. The main change is the introduction of an internal helper method for setting client assertion providers, which improves code clarity and enforces experimental feature validation. The unit tests are updated to explicitly enable experimental features to ensure correct test coverage.

Refactoring of client assertion registration:

• Added a new internal method WithClientAssertionInternal to encapsulate the logic for setting client assertion providers, and updated all public WithClientAssertion overloads to use this method. This also adds validation for the use of experimental features. (src/client/Microsoft.Identity.Client/AppConfig/ConfidentialClientApplicationBuilder.cs) [1] [2] [3] [4]

Test updates for experimental feature usage:

• Updated all relevant unit tests to call .WithExperimentalFeatures(true) when using WithClientAssertion, ensuring that tests reflect the required usage of experimental APIs. (tests/Microsoft.Identity.Test.Unit/PublicApiTests/ClientAssertionTests.cs, tests/Microsoft.Identity.Test.Unit/PublicApiTests/ConfidentialClientApplicationTests.cs) [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

Testing
Have unit, integration tests

Performance impact
none

Documentation

• All relevant documentation is updated.

[neha-bhargava]

Approve with comment