Introduce build minutes

What does this MR do?

It introduces build minutes for Shared Runners.

Assumptions:

• The build minutes are accounted per-namespace,
• You can overwrite amount of given build minutes per-namespace,
• You configure global amount of build minutes that namespace can spend,
• The amount of used build minutes is cleared out with separate cron job,
• It uses a separate table to store amount of build minutes spend.

The build minutes are updated after every build. It can't be done as per-pipeline event due to fact that builds from pipeline can be retried.

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

Why was this MR needed?

Screenshots (if relevant)

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 #1405 (closed)