feat: Add Custom Token Exchange Support

[tanya732]

Summary

This PR adds first-class support for the OAuth 2.0 Custom Token Exchange grant (urn:ietf:params:oauth:grant-type:token-exchange, RFC 8693) to the Authentication API

Changes

Custom Token Exchange

Added AuthAPI.exchangeToken(String subjectToken, String subjectTokenType), which returns a TokenRequest.

• Uses the OAuth 2.0 Token Exchange grant:
  urn:ietf:params:oauth:grant-type:token-exchange
• Hardcodes grant_type, consistent with the existing requestToken() and renewAuth() APIs.
• Accepts caller-supplied subject_token and subject_token_type.
• Requires client authentication (client secret or client assertion).
• Supports fluent .setAudience(...) and .setScope(...) configuration.

Example

TokenHolder tokens = authAPI
    .exchangeToken(
        "test-user123-john@example.com",
        "urn:mycompany:m2m-test-token"
    )
    .setAudience("https://YOUR_API_IDENTIFIER")
    .setScope("openid profile email")
    .execute()
    .getBody();