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

* feat: mattermost integration

* post review adjustments

packages/backend/src/apps/mattermost/common/set-base-url.ts

@@ -0,0 +1,9 @@
+import { TBeforeRequest } from '@automatisch/types';
+
+const setBaseUrl: TBeforeRequest = ($, requestConfig) => {
+  requestConfig.baseURL = $.auth.data.instanceUrl as string;
+
+  return requestConfig;
+};
+
+export default setBaseUrl;