ECR [request]: expose image labels in DescribeImages response

Hi!

**Tell us about your request**
I would like image labels to be exposed over the DescribeImages API. Docker labels are the only way to add immutable metadata to an image, having a way to view that data without pulling the image down would be really useful!

**Which service(s) is this request for?**
ECR

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We'd like to add metadata about a particular docker image to the image itself. Info such as: git hash, date built, repository, branch, etc; and then be able to lookup that data remotely using the ECR API.

This data is useful for working backwards from a container running in ECS, to the exact source that it was built from. Tags can serve a similar purpose but they're mutable and not connected directly to the image itself.

Thank you!