Add capabllity to return an operator message in the initial connection handshake

Substreams can already return the trace_id to the user.  Based on how that feature works, add a new feature to allow an additional message to be returned to the consumer.  

The operator could configure ` --common-connect-message: foobar`

Then  when user connects they could receive an "operator message: foobar".  Graph-node could log this information for debugging purposes.