feat: Adds Token Vault Support

[kailash-b]

Changes

Adds Token Vault support to AuthenticationController, allowing applications to exchange an Auth0 token (refresh or access token) for an external identity provider's access token (e.g. Google, GitHub, Slack) via the federated-connection access-token grant. The returned access token is the provider's own token - opaque to the application - and the grant returns no ID token and no refresh token.

• getTokenForConnectionWithRefreshToken(...) - exchange an Auth0 refresh token.
• getTokenForConnectionWithAccessToken(...) - exchange an Auth0 access token.
• getTokenForConnection(...) - generic escape hatch accepting a caller-supplied subject-token-type URN.

Docs updated: README.md (feature list) and EXAMPLES.md (new "Token Vault" section with usage and MCD guidance).

References

Testing

• This change adds test coverage
• This change has been tested on the latest version of Java 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

[tanya732]

LGTM