Introduce an `aws-lambda` feature. (#2035)

* Introduce an `aws-lambda` feature.

* Add CHANGELOG

* Use the new and shiny feature syntax to avoid creating an implicit `lambda_http` feature.

* Add `aws-lambda` feature to the Python server. Enable the `aws-lambda` feature in our example. Be explicit in providing an implementation of request rejection for Box<dyn Error>.

* Add an `aws-lambda` feature flag to the generated Python-specific crate. We enable it by default to make sure the Lambda method ends up visible in the final Python wrapper library.