Add AuthorizationGrantType.toString()

This adds AuthorizationGrantType.toString() which makes debuging easier.
In particular, it will help when performing unit tests which validate the
AuthorizationGrantType.

Issue gh-16382

oauth2/oauth2-core/src/main/java/org/springframework/security/oauth2/core/AuthorizationGrantType.java

@@ -111,4 +111,9 @@ public final class AuthorizationGrantType implements Serializable {
 		return this.getValue().hashCode();
 	}
 
+	@Override
+	public String toString() {
+		return "AuthorizationGrantType{" + "value='" + this.value + '\'' + '}';
+	}
+
 }