fix: x-amz-content-sha256 header is expected for unsigned payload and event stream operations

Issue \

Adds a new middleware when sigv4 is present that conditionally enables setting the SignedBodyHeader execution context attribute when the operation has the unsigned payload trait or is an event stream input. When set the signer will add the x-amz-content-sha256 header to the canonical request and set the header.
Adds the shouldSignHeader predicate to the AwsHttpSigner.Config. This was missing and allows auth scheme overrides to augment the set of signed headers. This allowed me to narrow down the issue since we include a few more headers than AWS CLI did. This should have been possible previously and I was surprised it wasn't.

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