Add support for COPY TO callback functions

This is useful as a way for extensions to process COPY TO rows in the
way they see fit (say auditing, analytics, backend, etc.) without the
need to invoke an external process running as the OS user running the
backend through PROGRAM that requires superuser rights.  COPY FROM
already provides a similar callback for logical replication.  For COPY
TO, the callback is triggered when we are ready to send a row in
CopySendEndOfRow(), which is the same code path as when sending a row
to a frontend or a pipe/file.

A small test module, test_copy_callbacks, is added to provide some
coverage for this facility.

Author: Bilva Sanaba, Nathan Bossart
Discussion: https://postgr.es/m/253C21D1-FCEB-41D9-A2AF-E6517015B7D7@amazon.com

src/backend/commands/copy.c

@@ -310,7 +310,7 @@ DoCopy(ParseState *pstate, const CopyStmt *stmt,
 
 		cstate = BeginCopyTo(pstate, rel, query, relid,
 							 stmt->filename, stmt->is_program,
-							 stmt->attlist, stmt->options);
+							 NULL, stmt->attlist, stmt->options);
 		*processed = DoCopyTo(cstate);	/* copy from database to file */
 		EndCopyTo(cstate);
 	}