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

* feat: mattermost integration

* post review adjustments

packages/backend/src/apps/mattermost/dynamic-data/index.ts

@@ -0,0 +1,3 @@
+import listChannels from './list-channels';
+
+export default [listChannels];

packages/backend/src/apps/mattermost/dynamic-data/list-channels/index.ts

@@ -0,0 +1,36 @@
+import { IGlobalVariable, IJSONObject } from '@automatisch/types';
+
+type TChannel = {
+  id: string;
+  display_name: string;
+};
+
+type TResponse = {
+  data: TChannel[];
+};
+
+export default {
+  name: 'List channels',
+  key: 'listChannels',
+
+  async run($: IGlobalVariable) {
+    const channels: {
+      data: IJSONObject[];
+      error: IJSONObject | null;
+    } = {
+      data: [],
+      error: null,
+    };
+
+    const response: TResponse = await $.http.get('/api/v4/users/me/channels'); // this endpoint will return only channels user joined, there is no endpoint to list all channels available for user
+
+    for (const channel of response.data) {
+      channels.data.push({
+        value: channel.id as string,
+        name: (channel.display_name as string) || (channel.id as string), // it's possible for channel to not have any name thus falling back to using id
+      });
+    }
+
+    return channels;
+  },
+};