Make AuthorizationGrantTypeConverter support custom grant type

Closes gh-10155

oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/jackson2/StdConverters.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -83,7 +83,7 @@ abstract class StdConverters {
 			if (AuthorizationGrantType.PASSWORD.getValue().equalsIgnoreCase(value)) {
 				return AuthorizationGrantType.PASSWORD;
 			}
-			return null;
+			return new AuthorizationGrantType(value);
 		}
 
 	}