[MessageHub]: Use Empty type rather than Node to detach a Node

Since the only information that we need for detaching an Node is the nodeId, and it is provided by GRPC metadata, we can use the Empty type rather than Node on detach method.
It depends on #163.

Start by reviewing issue #163 and the MessageHub detach method in the Python SDK. Trace how the nodeId is obtained from gRPC metadata and how the current Node request is handled. Done means the detach API uses Empty while preserving node identification through metadata, with the relevant tests updated and passing.

Written by the indexing model from the issue text.