Add Workspaces HTTP server in KAS

MR: Add workspaces http server (!2628 - merged)

Description

Add a new HTTP server in GitLab Agent Server(KAS). This HTTP server will route traffic to the appropriate agentw as part of Tunnel workspaces traffic to appropriate agentw (#688 - closed) .

As part of this issue, the goal is to simply introduce the plumbing for setting up the Workspaces HTTP server which will serve a constant response for now.