Implement /ext in Steve for Imperative API (#287)

This implements the Imperative API that is served at /ext with Steve. The imperative API is compatible with Kubernetes' API server and will be used as an extension API server.

scripts/test.sh

@@ -1,3 +1,13 @@
 #!/bin/bash
 
-go test ./...
+if ! command -v setup-envtest; then
+	echo "setup-envtest is required for tests, but was not installed"
+	echo "see the 'Running Tests' section of the readme for install instructions"
+	exit 127
+fi
+
+minor=$(go list -m all | grep 'k8s.io/client-go' | cut -d ' ' -f 2 | cut -d '.' -f 2)
+version="1.$minor.x"
+
+export KUBEBUILDER_ASSETS=$(setup-envtest use -p path "$version")
+go test ./...