Add support for pushdown of UNION queries when they all query the remote

From the Postgres Hackers Discord:

> `@theory`: In an FDW, is there a way to push down a union query when each select within it is from a foreign table?
>
> `@Robert Haas`: i'm not sure if it's possible, but if it is possible, i would guess that it involves `GetForeignUpperPaths`
>
> `@theory`: I need a tutorial for this stuff…
>
> `@tgl`: In principle we could support that I think, but I bet there's missing infrastructure that would make it hard. In the case of joins there's some in-core code that recognizes when all the joined tables belong to the same FDW, but I don't think `prepunion.c` has any equivalent. If you're excited about this I suggest working on a patch for that, rather than trying to figure out how to do it without any core support.
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> `@theory`: Thanks, super useful information. Will keep it in my mental backlog for sure as I work through learning the FDW stuff, see where there are gaps, and figure out which to try to plug first.