Fix INSERT EXEC with OUTPUT parameters

When a procedure with OUTPUT parameters is called via INSERT EXEC:
1. The OUTPUT parameter values should be returned to the caller variable
2. Only the SELECT results should be inserted into the target table
3. The OUTPUT parameter values should NOT be inserted into the table

The fix involves two key changes:

1. Copy OUTPUT parameter values to TopTransactionContext instead of
   the procedure's fn_cxt. The fn_cxt gets freed when the procedure
   returns, causing memory corruption (OID 0x7F7F7F7F error).

2. Set fcinfo->isnull = true to prevent the OUTPUT parameter values
   from being sent to the client/destination (which would insert them
   into the table). The new is_insert_exec_rewrite_active_hook in
   functioncmds.c allows this without causing 'procedure returned
   null record' error.

3. Register the hook in pl_handler.c to enable the INSERT EXEC
   context detection in PostgreSQL core.

This matches SQL Server behavior where OUTPUT parameters are returned
to caller variables but not inserted into the target table.

Author: monk0707