Add controller API client interface (#1467)

This PR adds an interface for controller API client, it allows us to
implement mock API client for unit testing with ease and we are
able to generate spec file without accessing the real Fission server.

pkg/fission-cli/cmd/support/resources/fissionversion.go

@@ -25,10 +25,10 @@ import (
 )
 
 type FissionVersion struct {
-	client *client.Client
+	client client.Interface
 }
 
-func NewFissionVersion(client *client.Client) Resource {
+func NewFissionVersion(client client.Interface) Resource {
 	return FissionVersion{client: client}
 }