Move Validatable/Defaultable interface checks out

[imjasonh]

These type checks are a signal that the types are validatable/defaultable, which is nice, but having them in the _types.go files makes it more difficult to consume those files by themselves.

There are no functiona changes expected.

As an alternative we could move these type checks into _test.go files, where consumers who only need the message types could just not consume the _test.go files. Or we could just remove the type checks entirely -- these types are still enforced when building cmd/controller and cmd/webhook.

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[dlorenc]

/approve

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[dlorenc]

/lgtm