Middy handler always requires an async handler

[minjie0501]

Describe the bug
The middy handler always expects the lambda handler to return void or Promise<any>. Now i have to make my handlers async even if they don't do any async tasks. I would like to use it with a synchronous handler.

To Reproduce
How to reproduce the behaviour:
Sample code:

import middy from "@middy/core";
import { APIGatewayProxyEvent, Context } from "aws-lambda";

const lambdaHandler = (event: APIGatewayProxyEvent, context: Context) => {
  return {
    statusCode: 200,
    body: JSON.stringify({
      message: "Hello world!",
      input: event,
    }),
  };
};

export const handler = middy(lambdaHandler);

Thrown error:
Type '{ statusCode: number; body: string; }' is not assignable to type 'void | Promise<any>'.

Expected behaviour
Middy should allow sync handlers

Environment (please complete the following information):

• Node.js: 18
• Middy: 4.6.1

[willfarrell]

Sounds like the types need to be updated. Like to put a PR together?

[cjbt]

Took a stab at this: #1102. Let me know if I'm missing anything.

[minjie0501]

Took a stab at this: #1102. Let me know if I'm missing anything.

Haven't really had time to take a look myself, thanks for the PR! 🙌