Proposal for solving api plugability

* In the conversion package, expose Src/Dest struct tags to conversionFuncs via the Scope, and the source/dest versions.
* In runtime, add "RawExtension" which just stores raw JSON (or yaml?) for deferred unpacking.
* In runtime, add global named Schemes.
* In runtime, add to the DefaultScheme conversionFuncs which convert between RawExtension and EmbeddedObject. They use a struct tag on the *internal* structure to determine which scheme to use, defaulting to DefaultScheme.

I think this is easy, readable, extensible, and will emit the JSON that we want.
The idea is that there can be multiple producers and consumers, the map lets everyone play nicely together. I'll fix the comments.