docs: add specification for ELF dlopen metadata

docs: add specification for ELF dlopen metadata

Tool to parse it and to use it for Debian packaging available at
https://github.com/systemd/package-notes

Add 3 levels of priority like Debian does, but using terminology
from RPM as it seems more apt.
We will very likely never use 'required', as we use this functionality
for optional features, but it is worth having it in the spec nonetheless
as we want it to be generalized.