Auth Manager API part 6: API enablement

[adutra]

6th and last PR for the Auth Manager API. Previous ones:

• Auth Manager API part 5: SigV4 Auth Manager #11995
• Auth Manager API part 4: RESTClient, HTTPClient #11992
• Auth Manager API part 3: OAuth2 Manager #11844
• Auth Manager API part 2: AuthManager #11809
• Auth Manager API part 1: HTTPRequest, HTTPHeader #11769

Once this PR is merged, the AuthManager API becomes effective.

Summary of changes:

• Minor changes to OAuth2Manager, mostly to properly handle empty auth sessions, as decided in Auth Manager API part 4: RESTClient, HTTPClient #11992;
• Enablement of SigV4 AuthManager and removal of HttpInterceptor code in HTTPClient and related tests;
• Enablement of AuthManager API in RESTSessionCatalog;
• Enablement of AuthManager API in credential refreshing code: VendedCredentialsProvider and OAuth2RefreshCredentialsHandler
  • OAuth2RefreshCredentialsHandler now caches its HTTPClient and AuthSession, instead of creating a new client and session for every refresh – pretty much like VendedCredentialsProvider already does.

[jbonofre]

LGTM, I just wonder if S3FileIO is not impacted (around the credentials).