Support RPM "extension tag" functionality

[dralley]

RPM has the concept of extension tags which do not exist within the header as entries in and of themselves, but rather they are constructed on the fly when requested if the user passed in the flags to allow them to be used.

https://github.com/rpm-software-management/rpm/blob/master/docs/manual/tags.md#extensions

These are basically just helpers which have been dropped into the same query system as the rest of the RPM tags.

I don't actually think we should handle this the same way, I would prefer we just add functions to the RPMPackageMetadata object for this stuff.