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

* feat: mattermost integration

* post review adjustments

packages/backend/src/apps/mattermost/auth/generate-auth-url.ts

@@ -0,0 +1,18 @@
+import { IGlobalVariable } from '@automatisch/types';
+import { URL, URLSearchParams } from 'url';
+import getBaseUrl from '../common/get-base-url';
+
+export default async function generateAuthUrl($: IGlobalVariable) {
+  const searchParams = new URLSearchParams({
+    client_id: $.auth.data.clientId as string,
+    redirect_uri: $.auth.data.oAuthRedirectUrl as string,
+    response_type: 'code',
+  });
+
+  const baseUrl = getBaseUrl($);
+  const path = `/oauth/authorize?${searchParams.toString()}`;
+
+  await $.auth.set({
+    url: new URL(path, baseUrl).toString(),
+  });
+}