ADR-001 Kubernetes first: the etcd subset the API server uses

QuantyKube implements the part of the etcd v3 API that the Kubernetes API server actually calls, on a single node, without Raft. A general etcd replacement for arbitrary clients is not the goal of this phase.

This is the shape kine has proven: k3s ships it in production, it implements a subset of the etcd API on top of SQL databases, and it does not cluster. That a store with these limits can carry a real Kubernetes control plane is therefore known, not hoped.

Raft is left out because writing it is a project of the same size as everything else here combined, and because a single node store is a form Kubernetes users already run and understand.

The cost: QuantyKube is unsuitable for general etcd use for now, and the README says so in its first section, as kine's does. Any etcd client other than the API server may call something that is not there. High availability of the store is not available at all.