Dynamic "config" system

[soxofaan]

Current config approaches in openeo-driver (and related) are static (e.g. env var based, or file based), and have to be updated with a full deploy pipeline (CI, deploy, promote, etc), which is perfectly fine for most things.

The client-credentials-to-user mapping is currently also a config item, but it is tedious that updates have to go through the deploy pipeline. This should better be moved to a more dynamic config system that can be updated on the fly (e.g. backed by Zookeeper or another DB). #245 would probably fit in this as well.

Some prior/related work of putting dynamic configs in zookeeper: eu-cdse/openeo-cdse-infra#33, Open-EO/openeo-geopyspark-driver#600

[soxofaan]

with Open-EO/openeo-geopyspark-driver#708 the client-credentials-to-user mapping is now practically not important anymore, so the importance of this ticket is pretty low at the moment