Added google androidpublisher/v3 api to verify google purchase tokens

server/google/client.go

@@ -0,0 +1,9 @@
+package google
+
+import (
+	"context"
+)
+
+type AndroidPublisherClient interface {
+	GetProductPurchase(ctx context.Context, packageName string, productId string, token string) (*ProductPurchase, error)
+}