Presigned S3 POST Object requests

### Describe the feature

It should be possible to create a presigned POST Object URL with the SDK. POST Object docs: https://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectPOST.html

### Use Case

Creating a presigned URL that can be used with a multi-part form in a web browser.

### Proposed Solution

_No response_

### 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
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Start with the linked Amazon S3 RESTObjectPOST documentation to understand the required presigned POST behavior and multipart form use case. The issue does not name SDK files, tests, or entry points. Done means the Rust SDK can create a presigned S3 POST Object URL usable by a web browser form, with suitable verification added.

Written by the indexing model from the issue text.