add types req res next

packages/slack/src/utils/verifying-is-slack-request.ts

@@ -1,12 +1,15 @@
 import { createHmac, timingSafeEqual } from 'crypto';
 import { stringify } from 'qs';
-
-
+import {Request, Response, NextFunction} from 'express'
 /**
    * Verify if the request came from slack
    * See: https://api.slack.com/authentication/verifying-requests-from-slack
    */
-export const verifyingIsSlackRequest = (req, res, next):string => {
+interface signingSecret extends Request{
+  signingSecret: string
+}
+
+export const verifyingIsSlackRequest = (req: Request & {signingSecret?:string}, res: Response, next: NextFunction) => {
 
   if (req.signingSecret == null) {
     return res.send('No signing secret.');