Add support for XEP-0321 (Remote Roster Management)

[fank]

This allows to inject roster information from external components, e.g. https://github.com/SpectrumIM/spectrum2.

Tested with 2.0.1beta2 Spectrum works fine with MongooseIM, but the problem is if you connect to a platform (e.g. ICQ or Steam) with multiple contacts, you need to add every friend to your buddy list on your own.

[fen-pl]

Hi @Fank

Thanks for the input. This extension is not in the roadmap for version 2.1.1 but we will consider it for 3.0.0. :)