[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.