Make Geo FDW configurable

What does this MR do?

User are able to set fdw: true in their config/database_geo.yml to use PostgreSQL FDW for Geo.

By default it is disabled, and if FDW isn't configured properly, it isn't used anyway.

Why was this MR needed?

It replaces gitlab-org/gitlab-ee!3990.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• 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?

gitlab-org/gitlab-ee!3990

Closes gitlab-org/gitlab-ee#4558.