RPM Support

There are few issues which inspired that refactor task:
- null value can come from creatrepo_c while in TextField it's recommended to use empty string in such cases https://pulp.plan.io/issues/4502
- more complicated structures which Pulp stores in a TextField requires serialization/deserialization (or maybe remodelling to make them simpler) https://github.com/pulp/pulp_rpm/pull/1298#discussion_r265235083
- there are two ways to get content into Pulp: via sync and via upload. And RPM plugin processes data differently in those cases, e.g. at upload time there is some data JSON serialization is happening, no serialization happens at sync time.

In general, if it's possible to isolate createrepo_c specifics (not only data conversion), code would be much better organized and readable.