Add support for parsing `repodata.json` file with `repodata_version` 2

[Hind-M]

Hi there,
Thanks for the great work!
There is this new CEP that landed with repodata_version 2, and it would be really cool if libsolv supports it to handle base_url.
I can also give it a try and open a PR if you want :)
Thanks!

[mlschroe]

I think the easiest way to implement this is to set the SOLVABLE_MEDIABASE property to the base_url if it is present.