slack/ecr-scanner-notifier: Add ECR scanner Slack notifier

[samkahchiin]

Summary

Send slack message when a vulnerability is found in the ECR image scanning

Motivation

closes #171

[swiknaba]

I like this idea. Using a Lambda seems to be a good choice. Any chance you'll have time to finish this? Low prior I think. Also: why add python (yet another tool) to our stack?

[swiknaba]

Nothing blocking, but if you have, time please ensure:

• the documentation in the README is not different from the default value in the variables file to avoid confusion
• never use [], unless a nil return value is a tested code path to reduce the number of possible bugs

I've also left a few NITs that help make the code more readable.

[swiknaba]

Awesome feature, thank you!

[swiknaba]

fixed the name of the lambda output :)