Add a setup.cfg with information from setup.py/setup()

[ana]

A lot of of the information from setup.py/setup() can be moved to a declarative setup.cfg file, see https://setuptools.pypa.io/en/latest/userguide/declarative_config.html

Background discussion at https://github.com/avocado-framework/avocado/discussions/5051

[ana]

Idea: migrate first the setup.py from all the optional_plugins, they should be easy to do.