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

[jlizen]

Related: #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.

[jlizen]

More context on usage of cargo lambda init: #1009 (comment)