campaigns: update the GraphQL schema to support SSH keys

Created by: LawnGnome

Right now, the GraphQL schema only handles personal access tokens as user credentials, and assumes that a user/code host combination can only have one credential. Code hosts that use SSH to clone and push will require users to hold two credentials, however: their PAT and the SSH key to use.

We need to extend our existing GraphQL queries and mutations to support this case. We also need to be able to detect that a code host uses SSH, which will likely require a new query.

Part of #16888 (closed).