Support AMI Watermark

Please search the existing issues for relevant feature requests, and use the
reaction feature
(https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/)
to add upvotes to pre-existing requests.

#### 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.

#### Description

AMI watermark feature is released.

#### Use Case(s)

Any relevant use-cases that you see.

#### Potential configuration

```hcl

source "amazon-ebs" "..." {
source_ami = ...

watermark {
name = ...
}

watermark {
name = ...
}

```

#### Potential References

https://aws.amazon.com/ko/about-aws/whats-new/2026/06/ec2-image-watermarks-allowed-images/
https://docs.aws.amazon.com/ko_kr/AWSEC2/latest/UserGuide/ami-watermark.html

Search existing issues for related AMI watermark requests, then read the amazon-ebs builder entry point and its tests to understand the current configuration flow. Review the linked AWS documentation and clarify the supported watermark configuration; done should include agreed behavior and project tests covering it.

Written by the indexing model from the issue text.