Add OAuth for traffic on Workspaces HTTP server

MRs:

• Configure workspaces http server on initialization (!2686 - merged)
• Add workspace authorize user access endpoint (!2771 - merged)
• Add OAuth logic for Workspaces HTTP Server (!2654 - merged)
• Add tests for workspaces auth middleware comple... (!2852 - merged)
• Fix workspaces transfer error proto validation ... (!2870 - merged)

Description

For all traffic arriving at the Workspaces HTTP server, we should perform an OAuth redirect. The details for performing this OAuth redirect should be lazily fetched from an internal endpoint in Rails which will create the GitLab instance-wide trusted OAuth application for Workspaces if one does not exists already and then return the required details.