[DO NOT MERGE] [Identity] CAE support.

[sadasant]

Draft for the CAE support.

Done so far:

• Translate the challenge code from other languages.
• Implement the onBeforeRequest method and the onChallenge method.
• Move Key Vault from the custom challenge authentication code to this core challenge authentication.
• Ensure existing tests pass for Key Vault.
• Add tests to the parsing of the challenges.

TODOs:

• Separate the changes on the BearerTokenAuthenticationPolicy into a new class, BearerChallengeAuthenticationPolicy.
• Hide CAE challenge details from the public API. Keep it generic.
• Test values with commas on the parsing of the challenges.
• Get Jeff's feedback ('cause core-http changes).
• Get Identity's feature crew's feedback.
• Write mocked tests for the CAE requests.
• Test this on the CAE champion scenarios.

Once approved (eventually):
Fixes #13156

[sadasant]

Adding [DO NOT MERGE] since we're pondering to release a patch first from a stable master, then include CAE and MSAL 2.0 in a beta release.

[sadasant]

Closed on favor of #13888