Fix aws-sdk-rust#930 (PutSnapshotBlock) (#3126)

## Motivation and Context
A regression in 0.56 caused `X-Amz-Content-sha256` to be omitted when
the `UnsignedPayload` trait was present. This re-enables the old
behavior and fixes this operation.
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->

## Description
Enable the `x-amz-content-sha256` header whenever the unsigned payload
trait is present.

Old code:
https://github.com/awslabs/smithy-rs/blob/smithy-rs-release-0.54.x/aws/sdk-codegen/src/main/kotlin/software/amazon/smithy/rustsdk/SigV4SigningDecorator.kt#L168

## Testing
- [x] Manual integration test
- [x] Modeled protocol test that ensures that header is set
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._