Support for async/await in handler

[lmammino]

Since version 0.3.0 we have support for async/await in middlewares.

We could potentially apply the same approach to the handler. Since the handler is invoked and managed by the middy wrapper, we could support different syntaxes including handlers returning promises and consequently async/await handlers.

These are the two syntaxes I envision might be cool to support:

const handler = middy((event, context) => {
  return Promise.resolve({
    statusCode: 200,
    body: 'hello'
  })
})

and, with async/await:

const handler = middy(async (event, context) => {
  return {
    statusCode: 200,
    body: 'hello'
  }
})

Any thoughts?

I think this is feasible by addressing the handler runtime logic here.