This commit adds two new helpers:
- util.experimentalflags.get_bool()
- util.experimentalflags.get_string()
similar to what we added in the images library in PR:
https://github.com/osbuild/images/pull/1248
The idea is that we provide experimentalflags for osbuild via
an environment like `OSBUILD_EXPERIMENTAL` and for those we
make no API promises. This will be initially used for better
debug of qemu-user.
