[CI / lambda-events] Ensure that `cargo lambda init`'s generated tests pass for all event types

Related: https://github.com/awslabs/aws-lambda-rust-runtime/issues/1009

We're playing a bit of whackamole with the tests that `cargo lambda init` generates failing in some cases. Currently it's due to spottiness on event type structs implementing `Default`. There presumably could be other reasons as well.

It would be great to have some sort of CI step that dynamically runs `cargo lambda init` against a feature matrix and makes sure the autogenned tests pass.

Start with the cargo lambda init command and its generated tests, reproducing failures across the event types and feature combinations described in the issue. Done means a CI step dynamically exercises those combinations and confirms that all generated tests pass.

Written by the indexing model from the issue text.