[RFE] Include optional extra dependencies in reqcheck

[amoralej]

Currently, reqcheck only compares requirements in requirements.txt file to Requires in the spec file.

It'd be also good to report optional dependencies (probably disabled by default by could be enabled with some cli option) to also include optional dependencies included in setup.cfg as extras. An example of implementation from RDO maintainer in:

https://github.com/jdennis/tools/blob/master/validate-python-deps-in-rpm#L222-L242

[apevec]

There are also optional deps in unexpected places e.g. in Ironic Agent https://opendev.org/openstack/ironic-python-agent/src/branch/master/plugin-requirements.txt