Allow `context` values to be special strings

[LecrisUT]

The context of this is for tmt, but the capability should be enabled in fmf broadly.

The idea is that some context values we would want to override the comparator, e.g. for distro we should be more explicit what it means and why distro < fedora-rawhide. For these we can then use https://github.com/rpm-software-management/fedora-distro-aliases to support aliases like fedora-development.

On the fmf side what can be done is to expose a registry which will use a specific class constructor for the context dict instead of keeping it as a str.