Add event definitions for CloudFormation custom resources

[chris-leach]

Issue #, if available:
Related to #680 - see below

Description of changes:
This PR adds event definitions for CloudFormation custom resource request objects. Requests are either a Create, Update, or Delete event, implemented as enum variants with internally tagged representation.

Note that, in addition to the fields documented in the links above, in practice CloudFormation sends a ServiceToken field in every request. Since this isn't documented I've encoded it as an Option<String> with #[serde(default)], so deserialisation will tolerate it being missing, but I can change this if desired.

---

This is intended as an initial building block of a framework for implementing Lambda custom resource handlers, of the kind requested in #680. These work by invoking a Lambda with one of the above event types, all of which include an S3 presigned URL to which a response must be posted. Failing to post a response can result in long delays, so tools to make it easy to send a response, and a fallback mechanism to send a failure response if the Lambda is about to time out, are desirable.

I'd appreciate feedback on whether further PRs to add this functionality (probably in a lambda_cfn crate) would be appreciated, but either way I think the event definitions are a useful addition.

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[calavera]

Thanks for adding those events!

I'd appreciate feedback on whether further PRs to add this functionality (probably in a lambda_cfn crate) would be appreciated, but either way I think the event definitions are a useful addition.

I'm hesitate to add more crates to this repository. I personally don't know much about CFN, so I would not be able to maintain that code. It'd probably be better if you want to create it and maintain it on your own.

[calavera]

Can you review the errors and fix them? 🙏

[chris-leach]

@calavera Done - I've removed the let ... else statements that I now realise were stabilised one release after the MSRV.

[calavera]

Just one more fix:

error: use of deprecated method `chrono::TimeZone::datetime_from_str`: use `DateTime::parse_from_str` instead
  --> lambda-events/src/custom_serde/codebuild_time.rs:21:13
   |
21 |         Utc.datetime_from_str(val, CODEBUILD_TIME_FORMAT)
   |             ^^^^^^^^^^^^^^^^^

[chris-leach]

@calavera This doesn't appear to be related to my changes - would you still like me to fix it in this PR?

[chris-leach]

@calavera pushed a fix for this - please review

[calavera]

LGTM, thanks!