Allow to configure KV store in the control plane via URI

[masci]

To be able to scale the control plane horizontally, the different instances should be able to share the same metadata storage. This is possible by passing an instance of a BaseKVStore to the control plane constructor in the state_store parameter, but it's not very practical because you would need some Python glue code to create the instance based on some configuration.

A better approach would be passing an URI describing the KVStore to be used in the control planes, so that each control plane replica could load it from an env var like CONTROL_PLANE_SERVICE_KV_STORE_URI.

Feature request coming from #357