text: Title: Local dev server
  Body: I think this would be mostly useful for developing Mangum itself, but it could be useful for application developers. Currently I've relied on [Serverless Offline](https://github.com/dherault/serverless-offline) to test deployments, but this requires a Serverless Framework configuration and gives me a lot more than I need for certain debugging tasks.
  
  Essentially what I think needs to be done is provide a layer to convert normal HTTP/WebSocket requests into AWS events and feed that to the adapter, then emulate the AWS response. Can probably create some tooling that can be re-used in the tests as well.