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/index.ts

@@ -0,0 +1,42 @@
+import defineAction from '../../../../helpers/define-action';
+import postMessage from './post-message';
+
+export default defineAction({
+  name: 'Send a message to channel',
+  key: 'sendMessageToChannel',
+  description: 'Sends a message to a channel you specify.',
+  arguments: [
+    {
+      label: 'Channel',
+      key: 'channel',
+      type: 'dropdown' as const,
+      required: true,
+      description: 'Pick a channel to send the message to.',
+      variables: true,
+      source: {
+        type: 'query',
+        name: 'getDynamicData',
+        arguments: [
+          {
+            name: 'key',
+            value: 'listChannels',
+          },
+        ],
+      },
+    },
+    {
+      label: 'Message text',
+      key: 'message',
+      type: 'string' as const,
+      required: true,
+      description: 'The content of your new message.',
+      variables: true,
+    },
+  ],
+
+  async run($) {
+    const message = await postMessage($);
+
+    return message;
+  },
+});