Add support for enabling the btree_gist pg extension

This extension is now required by gitlab as of gitlab-org/gitlab!32335 (merged)

Fixes: #2113 (closed)

Without it we see the following errors here in the chart during migrations:

psql:/srv/gitlab/db/structure.sql:5: ERROR:  permission denied to create extension "btree_gist"           │
│ migrations HINT:  Must be superuser to create this extension.                                                        │
│ migrations rake aborted!                                                                                             │
│ migrations failed to execute:                                                                                        │
│ migrations psql -v ON_ERROR_STOP=1 -q -X -f /srv/gitlab/db/structure.sql gitlabhq_production

This MR enables the extension in our included db. We will need to do followup to communicate the addition in the release notes, blog post, and documentation to ensure users who aren't using the bundled pg chart are aware.