[java] prototype, allow `@override` decorator apply to protocol method

Currently, only convenient method is affected by the `@override` decorator. Protocol method follows the REST API and is not affected.

We may see if we apply the decorator to protocol method as well, particularly the "grouping of parameters" (query and header).

One problem is that the parameters to protocol method only contains "required" parameters, while the parameters to convenient contains all. Also, protocol method "erases" the Enum and only retain String.