feat(mattermost): add auth and send message to channel action

* feat: mattermost integration

* post review adjustments

packages/backend/src/apps/mattermost/index.ts

@@ -0,0 +1,22 @@
+import defineApp from '../../helpers/define-app';
+import addAuthHeader from './common/add-auth-header';
+import addXRequestedWithHeader from './common/add-x-requested-with-header';
+import setBaseUrl from './common/set-base-url';
+import auth from './auth';
+import actions from './actions';
+import dynamicData from './dynamic-data';
+
+export default defineApp({
+  name: 'Mattermost',
+  key: 'mattermost',
+  iconUrl: '{BASE_URL}/apps/mattermost/assets/favicon.svg',
+  authDocUrl: 'https://automatisch.io/docs/apps/slack/connection',
+  baseUrl: 'https://mattermost.com',
+  apiBaseUrl: '', // there is no cloud version of this app, user always need to provide address of own instance when creating connection
+  primaryColor: '4a154b',
+  supportsConnections: true,
+  beforeRequest: [setBaseUrl, addXRequestedWithHeader, addAuthHeader],
+  auth,
+  actions,
+  dynamicData,
+});