Add tests for webhook and fix some webhook bugs (#33396)

This PR created a mock webhook server in the tests and added integration
tests for generic webhooks.
It also fixes bugs in package webhooks and pull request comment
webhooks.

services/webhook/general_test.go

@@ -319,8 +319,8 @@ func packageTestPayload() *api.PackagePayload {
 			AvatarURL: "http://localhost:3000/user1/avatar",
 		},
 		Repository: nil,
-		Organization: &api.User{
-			UserName:  "org1",
+		Organization: &api.Organization{
+			Name:      "org1",
 			AvatarURL: "http://localhost:3000/org1/avatar",
 		},
 		Package: &api.Package{