Expand on the CloudTrail event

[martinjlowm]

📬 Issue #, if available:
N/A

✍️ Description of changes:
The initial contribution of the CloudTrail event was not complete, but rather constructed from a handful of event examples. The structure is documented at:

• https://docs.aws.amazon.com/awscloudtrail/latest/userguide/cloudtrail-event-reference-user-identity.html

which indicates a number of optional fields and more fields that weren't already covered.

🔏 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.

[calavera]

It doesn't look like we have any tests for these events. Can you add some like we have in other event modules?

[calavera]

Thanks a lot for making these improvements!