Serve API version list, test with an integration test.

pkg/version/version.go

@@ -45,3 +45,9 @@ func Get() Info {
 func (info Info) String() string {
 	return info.GitVersion
 }
+
+// APIVersions lists the api versions that are available, to allow
+// version negotiation.
+type APIVersions struct {
+	Versions []string `json:"versions" yaml:"versions"`
+}