Add API extensions for facets
Add the ability to add facet's properties to host's show and list API requests.
Fixes #13216 - Added ability to extend API by creating facet subnode
This commit adds the ability for plugins to extend Host model with custom data that will be stored in a facet model.
In order to show this data an extension point is added to Host's view API, so the properties list can be extended by the facet.
Parts that are contained in this PR:
1. Added extension point to API V2: Now each facet will be able to add facet's properties to host's details
2. Added model extensions, now you can enumerate all active facets using Host#facets and Host#facets_with_definitions methods
3. Host#attributes will include the facets by default
4. Includes facets in Host#clone