Extend QGrpcOperation API to get the result metatype

As a user I want to know the underlying metatype for the QGrpcOperation result.
Current QGrpcOperation::read method allows reading the result message to any protobuf message, without the possibility for preliminary check for the result metatype.

Suggest adding API as such:

QMetaType QGrpcOperation::resultMetaType() const;

That will store the metatype of the RPC result. This also will unlock deserialization of messages to a pre-allocated by metatype QProtobufMessage without the need of strict typification in user code.

As a side task we may probably add the argument counter part for consistency:

QMetaType QGrpcOperation::argumentMetaType() const;

Extension:
QGrpcClientBase may also allow reading the resultMetaType(int methodIndex) by index for the respective methods in it's implementations.