feat(lambda-events): mark public structs/enums as #[non_exhaustive]

[mpindaru]

📬 Issue #1016

✍️ Description of changes:

🔏 By submitting this pull request

• I confirm that I've ran cargo +nightly fmt.
• I confirm that I've ran cargo clippy --fix.
• I confirm that I've made a best effort attempt to update all relevant documentation.
• I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ghost]

Good work! thanks!