GKG-webserver: add basic http service

Problem to Solve

We will need a long-running long-running KGK-webserver service for serving server-side requests (more details in the epic gitlab-org&17766).

Proposed Solution

• add a new http server (and a command to run this http server as a long-running service)
• there is already an http-server crate for client-side purposes, but server-side should be separate for now (create a new crate)
• in scope of this issue add a very basic /health endpoint for checking service status - this will be needed by zoekt-indexer for checking status of GKG services when sending heartbeats
• there is already @michaelangeloio's POC, a lot can be re-used for adding this endpoint