[lambda-events] Make sure that we have Default implemented for all event types

[jlizen]

In #1008, a contributor noticed that our kinesis event was missing its #[derive(Default)] and implemented that.

We should double check that no other events are missing Default impl as well. It's irritating to write unit tests without it.

[sdstolworthy]

To provide additional context here, cargo lambda init allows a user to select the type of event that will invoke their Lambda. All of the events in lambda_events are options. After completing the init flow, code is autogenerated in event_handler.rs that is specific to the event selected.

For example, for the KinesisEvent, the following test is generated in event_handler.rs:

#[cfg(test)]
mod tests {
    use super::*;
    use lambda_runtime::{Context, LambdaEvent};

    #[tokio::test]
    async fn test_event_handler() {
        let event = LambdaEvent::new(KinesisEvent::default(), Context::default());
        let response = function_handler(event).await.unwrap();
        assert_eq!((), response);
    }
}

Prior to #1008, this code failed to compile, because Default was not implemented on KinesisEvent.

A similar issue has been opened previously: #725.