Add utility to create/manage arn

### Describe the feature

In java sdk, we have class called [Arn](https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/arns/Arn.html) .

In rust sdk, we need something similar to create arn given serviceName and resourceName .

### Use Case

We construct arn manually for several reasons.

1. When assuming role, we need to construct full iam role arn and give it as input to sts.
2. In policy variables especially `${ec2:SourceInstanceArn}` , we need to construct full instance arn.

### Proposed Solution

May be, a new crate like `aws-sdk-arn` ?

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### A note for the community

### 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, please leave a comment

The issue names no repository files or tests. Start by examining the SDK's crate layout and the Java Arn reference linked in the issue, then determine where a Rust ARN utility belongs. Done should provide a supported way to construct full ARNs from a service name and resource name for the stated IAM, STS, and EC2 policy use cases.

Written by the indexing model from the issue text.