WIP: implement keep-alive requests for database connections

What does this MR do?

This implements a simple keep alive statement for the database connections. Every 10 minutes a Select 1 is being sent to the database.

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

Why was this MR needed?

In my company we have a firewall between our database server and our gitab server. By default it is configured to cut the connection after 2 hours of inactivity.
So it happend everyday that the firewall cut the connection in the middle of the night. This causes sidekiq to freeze completly and you have to manually restart sidekiq. This MR works around that by preventing the firewall from detecting an inactive connection.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #23885 (moved)