Separate entity URI/identity from hosting URL

[CasperWA]

(...) [F]or features I think it will be good in future update to separate the uri/identity of the entity and the url of the hosting service, as we will need to host entities created elsewhere with different namespaces. (...)

Originally posted by @quaat in #67 (review)

[CasperWA]

This will be done in a separate DataSpaces-dedicated entity service. This repository should then be split up as also mentioned in #137. One part of this repository will focus around the service-part, specifically the onto-ns.com-specific entity service.