Add origin readiness client (#374)

As described in #371, all services should implement a "readiness" endpoint, which checks whether the service is ready to serve traffic, i.e. whether it can reach all of its dependencies.

- Add a client for the origin's readiness endpoint.
- Additionally, add a ClusterClient (a client which abstracts which origin from the cluster is called).
- Rework the readiness test to use the client instead of directly calling the endpoint. Also, make each test case run separately (by using t.Run instead of just looping) to follow idiomatic Go.

lib/backend/constants.go

@@ -30,4 +30,5 @@ const (
 var (
 	ReadinessCheckNamespace string = core.NamespaceFixture()
 	ReadinessCheckName      string = core.DigestFixture().Hex()
+	ReadinessCheckDigest, _        = core.NewSHA256DigestFromHex(ReadinessCheckName)
 )