Backport authorized_keys

What

Backport to CE the use of authorized_keys from the database instead of a file. Also backports setting to turn on/off use of this and documentation.

Why

To simplify the work in helm charts where this will be the default. Identified in https://gitlab.com/charts/helm.gitlab.io/issues/13 and https://gitlab.com/charts/helm.gitlab.io/issues/14 as part of the ~"Cloud Native" and GCP Migration work.

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

• Does anything special need to be done when adding to ApplicationSettings?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Review
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database

Related

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/3953

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3959