[Go] Fix deepObject serialization that are anyOf (#19090)

* [Go] Fix deepObject serialization that are anyOf

Updates the go client generator to have the generated struct for anyOf types conform to `MappedNullable` interface.
Fixes query params serialization for deepObjects that are of `anyOf` type.

Implements the suggestion in https://github.com/OpenAPITools/openapi-generator/issues/19085

* check parameter style for deepObject serialization

* generate samples for go-echo-external-refs-test

* move test back to http port

* restrict to anyof models with discriminator

* update test

* add some tests

added some tests to `modules/openapi-generator/src/test/resources/3_0/go/petstore-with-fake-endpoints-models-for-testing-with-http-signature.yaml` and regenerated the samples

samples/client/petstore/go/auth_test.go

@@ -41,6 +41,8 @@ func TestOAuth2(t *testing.T) {
 		PhotoUrls: []string{"http://1.com", "http://2.com"}, Status: sw.PtrString("pending"),
 		Tags: []sw.Tag{{Id: sw.PtrInt64(1), Name: sw.PtrString("tag2")}}})
 
+	t.Log("Host is: ", client.GetConfig().Scheme+"://"+client.GetConfig().Host+", "+client.GetConfig().Servers[0].URL)
+
 	r, err := client.PetAPI.AddPet(context.Background()).Body(newPet).Execute()
 
 	if err != nil {