fix(lambda-events): impl Default for all Kinesis Event structs

[sdstolworthy]

📬 Issue #, if available:
I chatted with @jlizen after she opened #1009.

For testing ergonomics, it makes sense to implement Default on all of the structs in Kinesis Events. This PR is a follow up to #1008, and implements Default for all the KinesisEvent-related structs.

✍️ Description of changes:
Implement Default on all KinesisEvent-related structs.

I also changed the test that I merged in in the last PR. At first glance, this test feels unnecessary because it is not actually making any assertions. However, because cargo lambda init generates code that inherently relies on Default being implemented on these structs, I think a unit test that attempts to exercise that code path is worthwhile. Theoretically, the test can never fail at runtime, only compile time.

This test will protect against breaking cargo lambda init's autogenerated code in the absence of E2E tests.

🔏 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.

[jlizen]

Thanks!