Fixes TypeScript definition for http-router

[AntonioAngelino]

This PR allows the writing of handlers that use APIGatewayProxyEventV2 / APIGatewayProxyResultV2 like the following one:

async function postHandler(event: APIGatewayProxyEventV2, context: any): Promise<APIGatewayProxyResultV2> {
  return {
    statusCode: 200,
    body: JSON.stringify({ message: 'POST event submitted successfully', post: event.body }),
  }
}

I've also added a dedicated TS def test.

[willfarrell]

Awesome, thanks!