Alerting: Introduces /api/v1/ngalert/alertmanagers to expose discovered and dropped Alertmanager(s) (#37632)

* Alerting: Expose discovered and dropped Alertmanagers

Exposes the API for discovered and dropped Alertmanagers.

* make admin config poll interval configurable

* update after rebase

* wordsmith

* More wordsmithing

* change name of the config

* settings package too

pkg/tests/testinfra/testinfra.go

@@ -224,6 +224,13 @@ func CreateGrafDir(t *testing.T, opts ...GrafanaOpts) (string, string) {
 			_, err = featureSection.NewKey("enable", strings.Join(o.EnableFeatureToggles, " "))
 			require.NoError(t, err)
 		}
+		if o.NGAlertAdminConfigIntervalSeconds != 0 {
+			ngalertingSection, err := cfg.NewSection("ngalerting")
+			require.NoError(t, err)
+			_, err = ngalertingSection.NewKey("admin_config_poll_interval_seconds", fmt.Sprintf("%d", o.NGAlertAdminConfigIntervalSeconds))
+			require.NoError(t, err)
+		}
+
 		if o.AnonymousUserRole != "" {
 			_, err = anonSect.NewKey("org_role", string(o.AnonymousUserRole))
 			require.NoError(t, err)
@@ -265,12 +272,13 @@ func CreateGrafDir(t *testing.T, opts ...GrafanaOpts) (string, string) {
 }
 
 type GrafanaOpts struct {
-	EnableCSP            bool
-	EnableFeatureToggles []string
-	AnonymousUserRole    models.RoleType
-	EnableQuota          bool
-	DisableAnonymous     bool
-	CatalogAppEnabled    bool
-	ViewersCanEdit       bool
-	PluginAdminEnabled   bool
+	EnableCSP                         bool
+	EnableFeatureToggles              []string
+	NGAlertAdminConfigIntervalSeconds int
+	AnonymousUserRole                 models.RoleType
+	EnableQuota                       bool
+	DisableAnonymous                  bool
+	CatalogAppEnabled                 bool
+	ViewersCanEdit                    bool
+	PluginAdminEnabled                bool
 }