ClientRegistration.Builder defaulted to validating as an
authorization_code registration, though a custom grant type could be in
use. The actual grant_type is now verified for every case.
- Fixed validation in ClientRegistration.Builder
- New test that fails unless the issue is fixed.
Also made OAuth2AuthorizationGrantRequestEntityUtils public to help
implementing custom token response clients.
Fixes gh-7040
e7588fb32f