Return SSH key details in /internal/allowed response

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 gitlab-shell#203 (closed).

gitlab-shell changes: gitlab-shell!398 (merged)