Feature: Runtime Execution Simulator

[FullyTyped]

Name Pending.

Testing Lambda code can be difficult / unwieldy. While SAM exists for all supported languages, it is pretty limited in ergonomics, difficult to integrate with in CI, makes testing / infrastructure management unnecessarily complicated, and still requires further orchestration.

In an ideal world, we'd be able to "just" create a runtime with the corresponding service, add all the hooks, tracking, and tracing that we need, execute requests on top, and then verify the results, benchmark, get flamegraphs, etc, all without actually ever deploying, and all from a single tokio::test.

I have added a basic server in #1086, so this backlog issue is for extension / reuse of the implementation.

We should start from an internal implementation, and eventually publish a crate so that users can depend on it and integrate it in their CI.

[jlizen]

Another option would be to publish the simple simulator under a test-util feature. That might be a good first step since a standalone crate presumably might need more work to have a full feature set, more resilience, etc.

We could always cut such a feature over to using a standalone crate once available.