POC: [NX-496] Component metadata api

https://issues.sonatype.org/browse/NX-496
There was some discussion of this on the wiki. Basically these are generified GAV-like things. For maven, they map exactly, but for other types they don't, but there are rough analogs. The thinking as explained on the wiki was that this would help provide a sort of normalized view or other capabilities over all components in a repository, regardless of repository type. None of this is set in stone though...I think we'll need to play with it a bit and see what works.