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

* feat: mattermost integration

* post review adjustments

packages/backend/src/apps/mattermost/actions/send-a-message-to-channel/post-message.ts

@@ -0,0 +1,27 @@
+import { IGlobalVariable } from '@automatisch/types';
+
+type TData = {
+  channel_id: string;
+  message: string;
+};
+
+const postMessage = async ($: IGlobalVariable) => {
+  const { parameters } = $.step;
+  const channel_id = parameters.channel as string;
+  const message = parameters.message as string;
+
+  const data: TData = {
+    channel_id,
+    message,
+  };
+
+  const response = await $.http.post('/api/v4/posts', data);
+
+  const actionData = {
+    raw: response?.data,
+  };
+
+  $.setActionItem(actionData);
+};
+
+export default postMessage;