pgsql: 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(dot)dev(at)pm(dot)me>
Reviewed-By: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-By: Rohit Prasad <rohit(dot)prasad(at)arm(dot)com>
Reviewed-By: Michael Banck <mbanck(at)gmx(dot)net>
Reviewed-By: Manni Wood <manni(dot)wood(at)enterprisedb(dot)com>
Reviewed-By: Euler Taveira <euler(at)eulerto(dot)com>
Reviewed-By: Quan Zongliang <quanzongliang(at)yeah(dot)net>

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/f3c9e341cdf167ae3378e74e770558e81f9aa48e

Modified Files
--------------
doc/src/sgml/system-views.sgml | 28 ++++++
src/backend/catalog/system_views.sql | 4 +-
src/backend/commands/extension.c | 109 ++++++++++++++++-----
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 10 +-
.../t/001_extension_control_path.pl | 52 +++++++---
src/test/regress/expected/rules.out | 6 +-
src/tools/pgindent/typedefs.list | 2 +-
8 files changed, 165 insertions(+), 48 deletions(-)