Add support for client_credentials grant

Fixes gh-4982

config/src/test/java/org/springframework/security/config/annotation/web/configuration/OAuth2ClientConfigurationTests.java

@@ -98,6 +98,11 @@ public class OAuth2ClientConfigurationTests {
 			}
 		}
 
+		@Bean
+		public ClientRegistrationRepository clientRegistrationRepository() {
+			return mock(ClientRegistrationRepository.class);
+		}
+
 		@Bean
 		public OAuth2AuthorizedClientRepository authorizedClientRepository() {
 			return AUTHORIZED_CLIENT_REPOSITORY;