[ECR Public] [request]: Ability to select the default image that shows up on the public ecr gallery

### Community Note

* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue or have submitted a pull request, please leave a comment

**Tell us about your request**
On our public ECR repo when we open the main page there is an option to copy the URL of the default image. Usually this image belongs to the last uploaded image tag. Instead of that URL being based just on the last uploaded image can there be an option to mark a particular version as the default?

Customers doesn't have to worry about the version of the image that they have to choose. They will simply copy the URL available on the main page and do docker pull. The customer believe that this image is the latest image tag with all the latest features.

But take for example this public ECR repo: https://gallery.ecr.aws/appmesh/aws-appmesh-envoy and switch to **Image tags** tab. The default image url points towards v1.18.4.0-prod but in fact there is a newer version available v1.19.1.0-prod with much more features. Unless someone goes and read all the image tags they wouldn't know which is the default preferred latest version.

**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?**
This is a simple request. Now there is no option for us to specify which image can be tagged as default image. You can expose two options:
1. Mark the last uploaded image with a tag as the latest default
2. Or manually set which image tag will appear to be the default

Easily switch between these two options

**Are you currently working around this issue?**
Workaround is to delete the image and re-upload the latest version.

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

Reproduce the behavior in the linked public ECR repository, starting on its main page and the Image tags tab. Compare the copied default image URL with the available tags, including v1.18.4.0-prod and v1.19.1.0-prod. Done means users can choose whether the default is the latest uploaded tag or a manually selected image tag.

Written by the indexing model from the issue text.