settings: design the user flow when an SSH key needs to be provisioned

Created by: LawnGnome

When a user is adding a user credential to use with campaigns, they also need to configure SSH when the code host uses SSH to clone and push repositories. This will likely take the form of prompting the user during or after the modal where they enter their personal access token to copy the given public key and paste it into their code host settings as an authorised key.

Part of #16888 (closed).