Add paths of extensions to pg_available_extensions

Add paths of extensions to pg_available_extensions

Add a new "location" column to the pg_available_extensions and
pg_available_extension_versions views, exposing the directory where
the extension is located.

The default system location is shown as '$system', the same value
that can be used to configure the extension_control_path GUC.

User-defined locations are only visible for super users, otherwise
'<insufficient privilege>' is returned as a column value, the same
behaviour that we already use in pg_stat_activity.

I failed to resist the temptation to do a little extra editorializing of
the TAP test script.

Catalog version bumped.

Author: Matheus Alcantara <mths.dev@pm.me>
Reviewed-By: Chao Li <li.evan.chao@gmail.com>
Reviewed-By: Rohit Prasad <rohit.prasad@arm.com>
Reviewed-By: Michael Banck <mbanck@gmx.net>
Reviewed-By: Manni Wood <manni.wood@enterprisedb.com>
Reviewed-By: Euler Taveira <euler@eulerto.com>
Reviewed-By: Quan Zongliang <quanzongliang@yeah.net>