This PR adds code to make calls from boto clients directly to a Gateway without an HTTP server roundtrip. This can make internal calls much faster because they don't require IO.

The mechanism used to achieve this is botocore's before-send event, which can be used to short-circuit the request logic by returning a response from the event handler, before the HTTP request is even made.

I did some basic refactoring to make this easier, including:

• created localstack.runtime.components as a place for runtime-level singleton components
• added a new method to Gateway with the signature def handle(self, context: RequestContext, response: Response), which now makes the Gateway a drop-in replacement for a handler chain (and easier to call if you want to invoke it with a pre-existing RequestContext).

TODO:

• basic PoC implementation
• integration with the client factory and connect_to
• more benchmarks to see whether/where this is really useful
• more unit tests for different protocols
• documentation