Proposal for solving api plugability

Note that this is just a sketch and lacks test and not all details are implemented. That said, this proposal:

* In the conversion package, expose Src/Dest struct tags to conversionFuncs via the Scope.
* 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. They use a struct tag on the external versioned structure to determine what object version to emit.
* As an example of how this would look in practice, the VolumeSource field is updated to use this mechanism.

I think this is easy, readable, extensible, and will emit the JSON that we want.
This PR provides (most of) the magic. It doesn't provide any verification, that's got to be manual. It will currently reject types it doesn't know about, but I'll address that in a future PR with a pass-through type.