[PATCH] Expose port->authn_id to extensions and triggers

Hi all,

Stephen pointed out [1] that the authenticated identity that's stored
in MyProcPort can't be retrieved by extensions or triggers. Attached is
a patch that provides both a C API and a SQL function for retrieving
it.

GetAuthenticatedIdentityString() is a mouthful but I wanted to
differentiate it from the existing GetAuthenticatedUserId(); better
names welcome. It only exists as an accessor because I wasn't sure if
extensions outside of contrib were encouraged to rely on the internal
layout of struct Port. (If they can, then that call can go away
entirely.)

Thanks,
--Jacob

[1] https://www.postgresql.org/message-id/CAOuzzgpFpuroNRabEvB9kST_TSyS2jFicBNoXvW7G2pZFixyBw%40mail.gmail.com