Instead of pushing the responsibility of rescanning down to each different
kind of external table implement rescanning in fileam.c in a generic
fashion, by closing and reopening the underlying "url". This gets us rescan
support for custom and EXECUTE-type external tables, which was missing
before, and also makes the code simpler.

There are no known cases where the rescan support is currently needed (hence
no test case included), because the planner puts Materialize nodes on top of
external scans, but in principle every plan node is supposed to be
rescannable. I tested this by reverting the previous patch that fixed
using external scans in a subplan; without that patch, an external table
in a subplan would get rescanned.