[deployed] GKG-indexer: add basic http service

Problem to Solve

This is closely related to #188 (closed)

We will need a long-running long-running KGK-indexer service for serving server-side indexing 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) - same create as in #188 (closed) can be used. But both indexer and webserver services should run separately (not as single service). The reason is that we want to use different resource limits for both (related discussion)
• 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 we add a 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
• There is also another POC from @OmarQunsulGitlab : !254 (closed)