* #457 Duck type user annotation logic
* #457 Duck type user annotation logic - tests
* #457 Revert updater annotation key from lastModifier to updater
* #457 Rename HasSpec#GetSpec() to HasSpec#GetUntypedSpec()
* #457 Fix some indentation
* #457 Get group for user info annotations from the request
* #457 Reduce confusuion in webhook testing by using same group
This pulls the Knative webhook logic (oriented around the interfaces in `knative/pkg/apis`) into `knative/pkg`.
The code is largely copied as-is, with `keep.go` excluded. The main changes are to the test code, which in `knative/serving` still operate in terms of the serving types.
Fixes: https://github.com/knative/pkg/issues/9