Alerting: Add endpoint to revert to a previous alertmanager configuration (#65751)

* Alerting: Add endpoint to revert to a previous alertmanager configuration

This endpoint is meant to be used in conjunction with /api/alertmanager/grafana/config/history to
revert to a previously applied alertmanager configuration. This is done by ID instead of raw config
string in order to avoid secure field complications.

pkg/services/ngalert/api/api_alertmanager_test.go

@@ -381,6 +381,46 @@ func TestRouteGetAlertingConfigHistory(t *testing.T) {
 	})
 }
 
+func TestRoutePostGrafanaAlertingConfigHistoryActivate(t *testing.T) {
+	sut := createSut(t, nil)
+
+	t.Run("assert 404 when no historical configurations are found", func(tt *testing.T) {
+		req, err := http.NewRequest(http.MethodGet, "https://grafana.net", nil)
+		require.NoError(tt, err)
+		q := req.URL.Query()
+		req.URL.RawQuery = q.Encode()
+
+		rc := createRequestCtxInOrg(10)
+
+		response := sut.RoutePostGrafanaAlertingConfigHistoryActivate(rc, "0")
+		require.Equal(tt, 404, response.Status())
+	})
+
+	t.Run("assert 202 for a valid org and id", func(tt *testing.T) {
+		req, err := http.NewRequest(http.MethodGet, "https://grafana.net", nil)
+		require.NoError(tt, err)
+		q := req.URL.Query()
+		req.URL.RawQuery = q.Encode()
+
+		rc := createRequestCtxInOrg(1)
+
+		response := sut.RoutePostGrafanaAlertingConfigHistoryActivate(rc, "0")
+		require.Equal(tt, 202, response.Status())
+	})
+
+	t.Run("assert 400 when id is not parseable", func(tt *testing.T) {
+		req, err := http.NewRequest(http.MethodGet, "https://grafana.net", nil)
+		require.NoError(tt, err)
+		q := req.URL.Query()
+		req.URL.RawQuery = q.Encode()
+
+		rc := createRequestCtxInOrg(1)
+
+		response := sut.RoutePostGrafanaAlertingConfigHistoryActivate(rc, "abc")
+		require.Equal(tt, 400, response.Status())
+	})
+}
+
 func TestSilenceCreate(t *testing.T) {
 	makeSilence := func(comment string, createdBy string,
 		startsAt, endsAt strfmt.DateTime, matchers amv2.Matchers) amv2.Silence {