Simplify config getter and override system

[soxofaan]

This came up while trying to get Open-EO/openeo-geopyspark-driver#701 working:

configs defined in openeo-python-driver's OpenEoBackendConfig and used in openeo-python-driver can not be overriden from e.g. openeo-geopyspark-driver's gps_config_overrides as openeo-python-driver and openeo-geopyspark-driver have separate ConfigGetter instances and config_overrides only overrides one of them.

It probably pays of to simplify the config getter system a bit: just have a single ConfigGetter instance in the application. This implies that e.g. openeo-geopyspark-driver would overwrite the default one of openeo-python-driver.
Related: part of the simplification could also involve eliminating the subclass GpsConfigGetter