Right now when a client such as gitlab-shell calls the
`/api/v4/internal/allowed` API, the response only tells the client what
user has been granted access, and it's impossible to tell which deploy
key/token was used in the authentication request.

This commit adds `gl_key_type` (e.g. `deploy_key`) and `gl_key_id`
fields to the API response so that admins can identify the exact record
that was used to authenticate the user.

This is needed to support
https://gitlab.com/gitlab-org/gitlab-shell/-/issues/203.