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.
It's not implemented yet, but when it is, this will contain the JSON of the object that we couldn't understand.