RPM Support

All the use cases are described here: https://etherpad.net/p/modularity_use_cases

There are three types of modular content

- modulemd
- modulemd-defaults
- modular errata

Add possibility to manage all these modular types to pulp3.

For modulemd:
- sync
- publish
- copy (with artifacts, maybe (needs discussion) with module_defaults)
- recursive copy when depsolving is ready
- upload
- removal from a repo (with module artifacts)
- tracking a relationship between modulemd and its packages

For modulemd-defaults:
- sync
- publish
- ulpload
- simple copy/removal (provided by the core or general simple copy by rpm plugin)

For modular errata:
- sync
- publish
- upload
- simple copy/removal (provided by the core or general simple copy by rpm plugin)
- recursive copy when depsolving is ready