attach_data_session_metadata_decorator should decorate with instrument, visit_id and scan_number #666

This information is required for forwarding processing requests to central processing message bus, but is currently only available by decomposing other values. They should be passed explicitly and tested to ensure behaviour is consistent.

Acceptance Criteria

• When running a plan that has been decorated by the above decorator

• "visit" or "visit_id" is a key on the start document that contains the identifier of the visit (e.g. cm12345-6)
• "scan_number", "scan_identifier" or similar is also a key that contains the scan number (the number that is mutually increased with GDA)
• "instrument" is another key, and its value is the identifier of the beamline, e.g. "p45", "i20-1"