Writable FDW: returning clauses.

Hello.

While implementing the new writable API for FDWs, I wondered wether they
are any obvious problems with the following behavior for handling returning
clauses (for the delete case).

The goal is to fetch all required attributes during the initial scan, and
avoid fetching data on the delete operation itself.

- in the AddForeignUpdateTargets hook, add resjunk entries for the columns in
the returning clause
- in the ExecForeignDelete hook, fill the returned slot with values taken from
the planSlot.

--
Ronan Dunklau