Use clang-format for the generated code if the respective option is provided in qt_add_<protobuf|grpc> calls

We may offer users the post-formatting of protobuf/grpc code. This feature can be controlled by the optional argument of qt_add<protobuf|grpc>_ CMake methods:

qt_add_protobuf(mylib
    ...
    FORMAT
          clang-format
)

This will show users a nicer code when jumping using debugger or IDE.