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
* Initial set of files for //test
Currently there are no tests, but this will enable finishing setup Prow.
* Add build and unit tests
* Update README with instruction on how to run the unit tests