Add a "user_show_add_ssh_key_message" application setting for use with SSH certificates

What does this MR do?

This WIP merge request solves #49953 (closed). It should wait until !21009 (closed) is merged in, since it builds on top of it (to avoid a merge conflict, both modify the same docs).

Until !21009 (closed) is merged in the diff here is confusing, since it'll be a combination of two commits. This MR only discusses the second commit.

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

I have not actually tested or run any of this yet. Don't have a local instance I want to apply a DB migration to yet, but adding this from existing examples seemed relatively straightforward.

Why was this MR needed?

Solves #49953 (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
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#49953 (closed)