rgw/auth: don't require x-amz-content-sha256 in signed headers

aws sigv4 does not require s3 requests to sign x-amz-content-sha256
because it's already present in the canonical request as HashedPayload

Fixes: https://tracker.ceph.com/issues/79803

Signed-off-by: Casey Bodley <cbodley@redhat.com>