Add RestTemplate overloads to ClientRegistrations methods

[vpelikh]

This PR introduces overloaded methods in ClientRegistrations that accept a custom RestTemplate:

• fromOidcIssuerLocation(RestTemplate restTemplate, String issuer)
• fromIssuerLocation(RestTemplate restTemplate, String issuer)

Fixes #16833 #16920

Motivation

Currently, the discovery endpoints (OIDC and OAuth2) are called using a static RestTemplate with fixed connection/read timeouts. This limits customization for users who need to:

• Configure a proxy
• Set custom timeouts
• Add interceptors (e.g., for logging or authentication)
• Use a different HTTP client (e.g., with mutual TLS)

By providing overloads that accept a RestTemplate, users can now inject their own configured instance while reusing the existing discovery logic.

Changes

• Added two new public static methods as described above.
• The existing methods (fromOidcIssuerLocation(String) and fromIssuerLocation(String)) delegate to the new overloads using the default static RestTemplate.
• Internal oidc(...), oidcRfc8414(...), oauth(...) and getRfc8414Builder(...) methods were updated to accept a RestTemplate parameter; the existing static supplier-based approach remains unchanged.
• Added comprehensive tests for the new overloads, verifying:
  • Success scenarios (OIDC, OAuth2, and fallback flows)
  • Null RestTemplate or empty issuer handling
  • Error cases with proper exception propagation

Impact

• Backward compatible: existing callers are unaffected.
• Users can now leverage their own RestTemplate for discovery requests.

Testing

• All existing tests pass.
• New tests cover the added methods, using MockWebServer to simulate various discovery endpoints and responses.

Please review and let me know if any additional changes are needed.

[vpelikh]

Hey @jgrandja! Can you check this one?

[jgrandja]

@vpelikh We do not want to expose RestTemplate in the API.

Please see this comment as we plan to address this in the 7.2 release cycle.