feat(lambda-http): add example with tower_http crate

[nmoutschen]

Issue #, if available:

Description of changes:

This adds an example for the lambda-http crate using tower_http::cors::CorsLayer to inject CORS on responses.

I'm also taking that opportunity to add all features to the tower re-export, and clean some clippy warnings such as large variants for the LambdaRequest struct on lambda_http.

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[calavera]

please, don't make changes to the http event payloads, I'm about to replace them.

[nmoutschen]

please, don't make changes to the http event payloads, I'm about to replace them.

Agreed! I've reverted the changes in lambda-http/src.

[calavera]

nice example!