Create an interface for retrieving duck-type fields

[whaught]

Issue #1226

• This creates a new interface for receiving the core fields of KResource duck resources
• KResource implements the interface, but I intend to use this to implement for the basic API types
  • Unblocks the ability to cast to the interface and reason generically about the resource. I intend to use this to detect and run common logic for these resources.

[whaught]

/assign @vaikas @n3wscott

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-pkg-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
apis/duck/v1/kresource_type.go	Do not exist	50.0%

[whaught]

/hold I'm not sure if this is going to work without some reflection.

There isn't a way in golang to dynamically test if a resource implements an interface and then run logic on it that way.

The advantage of this would be instead of #1261 we'd reflect an instance of KRShaped from our type and from there we could create common logic (instead of duplicating that logic with the generator)

[whaught]

/unhold nah let's keep going with this. As long as we still create a new tag to genreconciler we'll only do this for KRShaped things. At least now we can create unit-testable functions on this type more easily instead of doing it all directly in the generated code.

[whaught]

/hold cancel

[n3wscott]

I think these come from RuntimeObject

[n3wscott]

/lgtm
/approve

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