GetCertificate allows us to start in TLS mode and dynamically fetch new certificates as they change. This will eventually allow us to decouple the cert creation process from the core webhook logic, and in a subsequent change service this from a secret lister cache.
* #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