add "admission" API group

This commit is an initial pass at providing an admission API group.
The API group is required by the webhook admission controller being
developed as part of https://github.com/kubernetes/community/pull/132
and could be used more as that proposal comes to fruition.

hack/verify-api-groups.sh

@@ -69,6 +69,7 @@ groups_without_codegen=(
 	"abac"
 	"componentconfig"
 	"imagepolicy"
+	"admission"
 )
 client_gen_file="${KUBE_ROOT}/cmd/libs/go2idl/client-gen/main.go"
 
@@ -92,6 +93,7 @@ done
 # them.  This happens for types that aren't served from the API server
 packages_without_install=(
 	"k8s.io/kubernetes/pkg/apis/abac"
+	"k8s.io/kubernetes/pkg/apis/admission"
 )
 known_version_files=(
 	"pkg/master/import_known_versions.go"
@@ -116,7 +118,7 @@ for expected_install_package in "${expected_install_packages[@]}"; do
 	done
 done
 
-# check all groupversions to make sure they're in the init.sh file.  This isn't perfect, but its slightly 
+# check all groupversions to make sure they're in the init.sh file.  This isn't perfect, but its slightly
 # better than nothing
 for external_group_version in "${external_group_versions[@]}"; do
 	if ! grep -q "${external_group_version}" "${KUBE_ROOT}/hack/lib/init.sh" ; then