Allow contrib/file_fdw to read from a program, like COPY FROM PROGRAM.

Allow contrib/file_fdw to read from a program, like COPY FROM PROGRAM.

This patch just exposes COPY's FROM PROGRAM option in contrib/file_fdw.
There don't seem to be any security issues with that that are any worse
than what already exist with file_fdw and COPY; as in the existing cases,
only superusers are allowed to control what gets executed.

A regression test case might be nice here, but choosing a 100% portable
command to run is hard.  (We haven't got a test for COPY FROM PROGRAM
itself, either.)

Corey Huinker and Adam Gomaa, reviewed by Amit Langote

Discussion: <CADkLM=dGDGmaEiZ=UDepzumWg-CVn7r8MHPjr2NArj8S3TsROQ@mail.gmail.com>