Improve env variable interpolation

# Description

While searching for a nice way to set configuration settings with env variables I searched a little bit in the documentation and struggled somehow with the current limitations.

While it is possible to read some (so far for the default configuration unspecified) values directly from the env variables, it doesn't allow us to set variables like the `server:url` from the env variables due to some default value that is set here https://github.com/geopython/pywps/blob/main/pywps/configuration.py#L76

However, the documentation also pointed at some point to the pycsw project that allows some basic env variable interpolation https://docs.pycsw.org/en/latest/configuration.html#using-environment-variables-in-configuration-files

My idea now is to copy this functionality & use its EnvInterpolation class for the config parser.