Alerting: Remove url based external alertmanagers config (#57918)

* Remove URL-based alertmanagers from endpoint config

* WIP

* Add migration and alertmanagers from admin_configuration

* Empty comment removed

* set BasicAuth true when user is present in url

* Remove Alertmanagers from GET /admin_config payload

* Remove URL-based alertmanager configuration from UI

* Fix new uid generation in external alertmanagers migration

* Fix tests for URL-based external alertmanagers

* Fix API tests

* Add more tests, move migration code to separate file, and remove possible am duplicate urls

* Fix edge cases in migration

* Fix imports

* Remove useless fields and fix created_at/updated_at retrieval

Co-authored-by: George Robinson <george.robinson@grafana.com>
Co-authored-by: Konrad Lalik <konrad.lalik@grafana.com>

pkg/services/ngalert/api/api_configuration_test.go

@@ -18,23 +18,13 @@ func TestExternalAlertmanagerChoice(t *testing.T) {
 	tests := []struct {
 		name               string
 		alertmanagerChoice definitions.AlertmanagersChoice
-		alertmanagers      []string
 		datasources        []*datasources.DataSource
 		statusCode         int
 		message            string
 	}{
-		{
-			name:               "setting the choice to external by passing a plain url should succeed",
-			alertmanagerChoice: definitions.ExternalAlertmanagers,
-			alertmanagers:      []string{"http://localhost:9000"},
-			datasources:        []*datasources.DataSource{},
-			statusCode:         http.StatusCreated,
-			message:            "admin configuration updated",
-		},
 		{
 			name:               "setting the choice to external by having a enabled external am datasource should succeed",
 			alertmanagerChoice: definitions.ExternalAlertmanagers,
-			alertmanagers:      []string{},
 			datasources: []*datasources.DataSource{
 				{
 					OrgId: 1,
@@ -51,7 +41,6 @@ func TestExternalAlertmanagerChoice(t *testing.T) {
 		{
 			name:               "setting the choice to external by having a disabled external am datasource should fail",
 			alertmanagerChoice: definitions.ExternalAlertmanagers,
-			alertmanagers:      []string{},
 			datasources: []*datasources.DataSource{
 				{
 					OrgId:    1,
@@ -66,7 +55,6 @@ func TestExternalAlertmanagerChoice(t *testing.T) {
 		{
 			name:               "setting the choice to external and having no am configured should fail",
 			alertmanagerChoice: definitions.ExternalAlertmanagers,
-			alertmanagers:      []string{},
 			datasources:        []*datasources.DataSource{},
 			statusCode:         http.StatusBadRequest,
 			message:            "At least one Alertmanager must be provided or configured as a datasource that handles alerts to choose this option",
@@ -74,7 +62,6 @@ func TestExternalAlertmanagerChoice(t *testing.T) {
 		{
 			name:               "setting the choice to all and having no external am configured should succeed",
 			alertmanagerChoice: definitions.AllAlertmanagers,
-			alertmanagers:      []string{},
 			datasources:        []*datasources.DataSource{},
 			statusCode:         http.StatusCreated,
 			message:            "admin configuration updated",
@@ -82,7 +69,6 @@ func TestExternalAlertmanagerChoice(t *testing.T) {
 		{
 			name:               "setting the choice to internal should always succeed",
 			alertmanagerChoice: definitions.InternalAlertmanager,
-			alertmanagers:      []string{},
 			datasources:        []*datasources.DataSource{},
 			statusCode:         http.StatusCreated,
 			message:            "admin configuration updated",
@@ -94,7 +80,6 @@ func TestExternalAlertmanagerChoice(t *testing.T) {
 		t.Run(test.name, func(t *testing.T) {
 			sut := createAPIAdminSut(t, test.datasources)
 			resp := sut.RoutePostNGalertConfig(ctx, definitions.PostableNGalertConfig{
-				Alertmanagers:       test.alertmanagers,
 				AlertmanagersChoice: test.alertmanagerChoice,
 			})
 			var res map[string]interface{}