fix: expose CloudWatchMetricAlarm and CloudWatchCompositeAlarm

[junlarsen]

📬 Issue #, if available:
N/A

✍️ Description of changes:
These types are referred to in the rust documentation for the CloudWatchAlarm type, but they were previously not exported by the crate, and instead left unused.

🔏 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]

Thanks!