Create VSCode AuthenticationProvider extension for GitLab Workspaces

MR: Pending

Description

As a workspaces user, I want to expose the workspace's GitLab Authentication token to VSCode so that I can use it to persist my settings using the Settings Sync feature.

Acceptance Criteria

• The AuthenticationProvider provides a single user session which cannot be removed.
• The AuthenticationProvider doesn't allow creating another user session.
• The AuthenticationProvider exposes the GitLab's user name in the VSCode user sessions UI