Implement gRFC A97: xDS JWT Call Credentials

[khush2520]

Implements gRFC A97 to support file-based JSON Web Token (JWT) Call Credentials for xDS-enabled clients. Implemented using an experimental coding agent.
Summary of Changes

• Core JWT Capabilities: Added JwtTokenFileCallCredentials in grpc-auth for lazy-loading, caching, exp-claim validation, and asynchronous token refresh via ExponentialBackoffPolicy.
• xDS Bootstrap Parser Integration: Updated BootstrapperImpl and ServerInfo in grpc-xds to parse call_creds JSON lists, combining them utilizing composite credentials. Flag-guarded securely behind GRPC_EXPERIMENTAL_XDS_BOOTSTRAP_CALL_CREDS.
• Testing: Added rigorous unit tests and an end-to-end XdsJwtCallCredsIntegrationTest verifying real client-to-server TLS + JWT Authorization transmission.

[linux-foundation-easycla]

• ❌ - login: @khush2520 / name: Khooshi Asmi. The commit (e5863c6, 6bc5e3e, 86b77fb, c97e6b5) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please visit our EasyCLA portal and chat with our support bot.