Improve forward/backward compatibility of OpenEoBackendConfig

[soxofaan]

Current OpenEoBackendConfig system is quite strict at the moment:
passing unknown config values to the constructor cause an "unexpected keyword argument" exception. This is fine as default behavior (e.g. to catch config value typo's as early as possible), but can cause problems in practice. For example, sometimes we have to run older images (that don't know yet about config field X) with newer configs (that define config X). It's not ideal that this fails as hard as it currently does.

It should be possible to skip unknown config values (and trigger warning/error logs instead of failing hard)