Allow all deploy keys to be retrieved via API, regardless of project affiliation.

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/16920

Administrators may need to 'audit' the system to see what public keys are present. It's currently not possible to obtain all deploy keys that are in the system, except to iterate over all projects and try to compare the deploy keys. Would it be possible to add an endpoint to retrieve all keys, regardless of project affiliation?

cc/ @rspeicher Thoughts?