campaigns: extend settings UI to allow for SSH keys to be provisioned

Created by: LawnGnome

This is the implementation ticket for #16892 (closed), and requires #16893 (closed), #16890 (closed), and #16891 (closed) to be completed.

At a high level: we need to be able to prompt users with SSH-based code hosts to add a public key to their code host settings when they're adding a PAT for that code host.

Part of #16888 (closed).