Add support for Resource Owner Password Credentials grant

Fixes gh-6003

samples/boot/oauth2webclient-webflux/src/main/java/sample/config/WebClientConfig.java

@@ -54,6 +54,7 @@ public class WebClientConfig {
 						.authorizationCode()
 						.refreshToken()
 						.clientCredentials()
+						.password()
 						.build();
 		DefaultServerOAuth2AuthorizedClientManager authorizedClientManager =
 				new DefaultServerOAuth2AuthorizedClientManager(

samples/boot/oauth2webclient/src/main/java/sample/config/WebClientConfig.java

@@ -52,6 +52,7 @@ public class WebClientConfig {
 						.authorizationCode()
 						.refreshToken()
 						.clientCredentials()
+						.password()
 						.build();
 		DefaultOAuth2AuthorizedClientManager authorizedClientManager = new DefaultOAuth2AuthorizedClientManager(
 				clientRegistrationRepository, authorizedClientRepository);