[Iteration 1] Add API to receive task requests from indexer

Problem to solve

This is an issue for implementing the API for [Iteration 1] Zoekt Indexer: Implement heartbea... (#424115 - closed)

Proposal

As part of this issue we need to implement a new API to receive heartbeats from the indexer.

We need to decide if it will be:

• Internal API
• Similar to how Runner API is implemented (I believe it's api/v4 with auth tokens)

One caveat is that only instance admins should be able to register a node. In case of Personal access tokens, we don't want anyone to be able to add one.

It needs to be protected by authentication. On top of that, we'll need to expand Zoekt::Shard. I propose: #424456 (closed)