pgsql: add initial support to CopyIn mode/subproto

pgsql: add initial support to CopyIn mode/subproto

This sub-protocol inspects messages sent mainly from the frontend to
the backend after a 'COPY FROM STDIN' has been processed by the
backend.

Parses new messages:
- CopyInResponse -- initiates copy-in mode/sub-protocol
- CopyData (In) -- data transfer message, from frontend to backend
- CopyDone -- signals that no more CopyData messages will be seen from
  the frontend, for the current transaction
- CopyFail -- used by the frontend to signal some failure to proceed
  with sending CopyData messages

Task #7645