Feat: Add validations to SNS Receiver in Alertmanager Config CR (#8217)

* api: update variable type

to match the type of optional and add more validation

* am-validation: add url validation and test case

to semantically verify URL

* alertmanager: move test case to operator

to validate at the correct part

* validation: update validation method and error message

for a clear validation and clean code

* validation: update validationSnsConfig method

to let the compiler guess variable type

* validation: update import order

to fix golang lint check

* trigger ci

test/e2e/alertmanager_test.go

@@ -1204,7 +1204,7 @@ func testAlertmanagerConfigCRD(t *testing.T) {
 				}},
 				SNSConfigs: []monitoringv1alpha1.SNSConfig{
 					{
-						ApiURL: "https://sns.us-east-2.amazonaws.com",
+						ApiURL: ptr.To(monitoringv1alpha1.URL("https://sns.us-east-2.amazonaws.com")),
 						Sigv4: &monitoringv1.Sigv4{
 							Region: "us-east-2",
 							AccessKey: &v1.SecretKeySelector{
@@ -1220,7 +1220,7 @@ func testAlertmanagerConfigCRD(t *testing.T) {
 								Key: testingSecretKey,
 							},
 						},
-						TopicARN: "test-topicARN",
+						TopicARN: ptr.To("test-topicARN"),
 					},
 				},
 				WebexConfigs: []monitoringv1alpha1.WebexConfig{{