Add conditional writes

Amazon S3 now supports conditional writes ([blog post](https://aws.amazon.com/about-aws/whats-new/2024/08/amazon-s3-conditional-writes/?ck_subscriber_id=585917945)). [Docs](https://docs.aws.amazon.com/AmazonS3/latest/userguide/conditional-requests.html#conditional-error-response) indicate that `s3:PutObject` and `s3:CompleteMultipartUpload` now support the request header `If-None-Match: *`, in which case S3 will:
1. return a 200 if it created the object did not exist and was created as requested,
2. return a 412 Precondition Failed if the object already exists, or
3. return a 409 Conflict if the object is deleted before creation completes

This upstream change updates #218.