Include extension path on pg_available_extensions

Hi all,

On [1] it was mentioned that it could be a good idea to include the
extension location when listening the available extensions on
pg_available_extensions to make it clear to the user the location of an
extension that Postgres is seeing based on the extension_control_path
GUC.

The attached patch implements this idea. Extensions installed on $system
path will not show the actual value of the $system macro and it will
show the macro itself, for example:

postgres=# show extension_control_path;
extension_control_path
---------------------------------------------------
/usr/local/my/extensions/share/postgresql:$system
(1 row)

postgres=# select * from pg_available_extensions;
name | default_version | installed_version | comment | location
---------+-----------------+-------------------+--------------------------------------------------+---------------------------------------------------
envvar | 1.0.0 | | Get the value of a server environment variable | /usr/local/my/extensions/share/postgresql/extension
amcheck | 1.5 | | functions for verifying relation integrity | $system
bloom | 1.0 | | bloom access method - signature file based index | $system

I'm not sure if this should be included on 18 release since this is not
a bug fix but an improvement on the extension system by itself.

Any opinions on this?

[1] https://www.postgresql.org/message-id/CAKFQuwbR1Fzr8yRuMW%3DN1UMA1cTpFcqZe9bW_-ZF8%3DBa2Ud2%3Dw%40mail.gmail.com

--
Matheus Alcantara