Make GL_ID format consistent

ZD Reference: https://gitlab.zendesk.com/agent/tickets/43481

Our git hooks are currently relying on the GL_ID variable passed in by GitLab to determine the username for additional authentication. We access the user info via the internal/discover API method which takes a key_id parameter.

However, the issue we currently have is that the GL_ID is received as key-# when pushing via SSH, but user-# when pushing via UI (eg. merge request) or via API methods (eg. creating a branch).

We would like the GL_ID to either be consistent – i.e. always key-# -- or the internal/discover API method to also accept a user_id parameter.