Add a new identifier for deploy token

What does this MR do and why?

This MR is part of the effort to fix Git Streaming Audit Events 404/401 errors for non-human actors in HTTP requests.

When a git push/pull is performed using a DeployToken, the audit event fails because the gl_id is incorrectly
generated as user-{id} instead of deploy-token-{id}. This causes the audit endpoint to fail when trying to look up the actor.

This MR updates Gitlab::GlId.gl_id to return type-specific identifiers:

• User → user-{id}
• DeployToken → deploy-token-{id}
• Key → key-{id} (also covers DeployKey via inheritance)

This is a prerequisite for #570235 (closed), which will update Workhorse to send this identifier to the audit endpoint.

Note on Key support

The original issue suggested adding Key support to gl_id. After investigation, gl_id is
called from four places, and none of them pass a Key:

1. call site: lib/gitlab/workhorse.rb

• what's passed: user
• why not Key: comes from authenticated_user which returns User or DeployToken from HTTP
  auth

2. call site: ee/app/controllers/ee/repositories/git_http_controller.rb

• what's passed: user
• why not Key: same as above, even the Geo override returns deploy_key.user (User), not the key itself

3. call site: lib/api/internal/base.rb

• what's passed: actor.user
• why not Key: GitAccessActor sets @user = key.user, so it's always a User

4. call site: lib/gitlab/git/user.rb

• what's passed: gitlab_user
• why not Key: all callers pass current_user or similar User objects

References

Issue: #570234 (closed)

Epic: gitlab-org#20506

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