WIP: Persist protected flag on Ci::Runner model

What does this MR do?

This MR adds a protected flag on Persist protected flag on Ci::Runner model and expose it on UI for on/off the flag.

This MR is a before step of "Protected runner executes jobs on protected branch. Unprotected runner executes jobs on unprotected branch". Because the MR will meet a technical challenge.

Solution 1 => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13194 Solution 2 => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13216

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, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• 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
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Related https://gitlab.com/gitlab-org/gitlab-ce/issues/33281