RPM Support

repomd.xml references all the repository metadata which is actual and should be used.
Pulp recognizes only a subset of those and processes them in some way.
Also, any rpm repo provider can add metadata of custom type.

Pulp needs to support custom/unknown repository metadata:
- it should be a separate content type
- it can be synced/published/copied/uploaded
- there can be only one item of any type of repository metadata in a repo version (e.g. only one content of type "product_id")
- it should be immutable in Pulp (this content unit can;t be modified, only new one can be created and added to a repo and the old one should be removed)