Support 'active' setting on Runner Registration API endpoint

What does this MR do?

Adds support for configuring the active setting of Runner, during registration.

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

Why was this MR needed?

GitLab API change required for gitlab-runner#2045 (closed).

Screenshots (if relevant)

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
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Related to gitlab-runner#2045 (closed)