refactor: Adjust send a message to channel to use slack client

packages/backend/src/apps/slack/client/endpoints/post-message-to-channel.ts

@@ -0,0 +1,34 @@
+import SlackClient from '../index';
+
+export default class PostMessageToChannel {
+  client: SlackClient;
+
+  constructor(client: SlackClient) {
+    this.client = client;
+  }
+
+  async run(channelId: string, text: string) {
+    const headers = {
+      Authorization: `Bearer ${this.client.connection.formattedData.accessToken}`,
+    };
+
+    const params = {
+      channel: channelId,
+      text,
+    };
+
+    const response = await this.client.httpClient.post(
+      '/chat.postMessage',
+      params,
+      { headers }
+    );
+
+    if (response.data.ok === 'false') {
+      throw new Error(
+        `Error occured while posting a message to channel: ${response.data.error}`
+      );
+    }
+
+    return response.data.message;
+  }
+}