Add Nullable attributes to events for kotlin interop

APIGatewayEvents (and possibly other types) aren't annotated as having nullable fields, leading to confusing null pointer exceptions from the kotlin side of the world. Proper annotations would make it less painful to use `aws-lambda-java-libs` for developers, since it's impossible to know without trial and error what is or can be null.

Start by locating APIGatewayEvents and reviewing the other event types in aws-lambda-java-libs to determine which fields can be null. Check how the project currently represents nullability and how Kotlin consumes these Java classes. Done means nullable fields are consistently annotated across the relevant event types and Kotlin callers no longer need to discover nullability through runtime failures.

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