Allow source locators as plugins

[schaefi]

Problem description

Source locators like obs:// are service specific coding to meet the requirements of the service.
Adding more of them like suggested in #1809 would increase the kiwi core code with service
specific code and dependencies. The idea is to refactor the URI class in kiwi such that it
can be easily extended by the existing kiwi plugin infrastructure.

As a first task the obs:// URI code could be moved and provided as a plugin package