Invent pg_hba_file_rules view to show the content of pg_hba.conf.

Invent pg_hba_file_rules view to show the content of pg_hba.conf.

This view is designed along the same lines as pg_file_settings, to wit
it shows what is currently in the file, not what the postmaster has
loaded as the active settings.  That allows it to be used to pre-vet
edits before issuing SIGHUP.  As with the earlier view, go out of our
way to allow errors in the file to be reflected in the view, to assist
that use-case.

(We might at some point invent a view to show the current active settings,
but this is not that patch; and it's not trivial to do.)

Haribabu Kommi, reviewed by Ashutosh Bapat, Michael Paquier, Simon Riggs,
and myself

Discussion: https://postgr.es/m/CAJrrPGerH4jiwpcXT1-46QXUDmNp2QDrG9+-Tek_xC8APHShYw@mail.gmail.com