Extend Geo Node Status with information in replication slots

Exposes a new model PgReplicationSlot that can query information on PostgreSQL replication slots.

The number of used/unused/total slots can be queried, as well as the number of bytes that, overall, the replication slots are reserving in WAL (indicating there could potentially be a problem).

These values are also added to the GeoNodeStatus object.

Are there points in the code the reviewer needs to double check?

The PgReplicationSlot model

Why was this MR needed?

We want to expose more information about replication slots in the Geo status screen, and this is the first part of that.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• API support added - coming in follow on MR
• Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Related to #4179 (closed)