Add formatter for Amazon Managed Prometheus alerts

[jfreeland]

Is your request related to a new offering from AWS?

Is this functionality available in the AWS provider for Terraform? See CHANGELOG.md, too.

• Yes ✅: please list the AWS provider version which introduced this functionality

Is your request related to a problem? Please describe.

No.

Describe the solution you'd like.

CAUTION: I suspect this might not be the right place for this nor the best approach.

I am using this function for sending CloudWatch alarms to Slack. I wanted to be lazy and piggyback on using the same topic and function to send AMP alarms to Slack. It technically works as intended, but could potentially be a touch prettier.

Describe alternatives you've considered.

Using a separate function.

Additional context

[antonbabenko]

Thank you for the request of this important feature. You (or anyone else) are welcome to open pull-requests with such additions. Look into existing formatters for examples.