Add pgvector extension

What does this merge request do and why?

Adds pgvector to be used by the new embedding database.

Relates to gitlab#404396.

How to set up and validate locally

1. Ensure pgvector is enabled by adding the below configuration in gdk.yml:

---
pgvector:
  enabled: true

1. Check out to this merge request's branch.
2. Run gdk reconfigure.
3. Run ls -l $(pg_config --pkglibdir)/vector.so to verify the extension file was installed.
4. Run gdk psql.
5. In psql, run: CREATE EXTENSION vector;
6. In psql, run: \dx and see that the extension was added.
7. In psql, run: DROP EXTENSION vector; to remove it to clean up since it will be used in a different database, not the gitlabhq_development database.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.