aws_sigv4: consult x-%s-content-sha256 for payload hash

aws_sigv4: consult x-%s-content-sha256 for payload hash

`Curl_output_aws_sigv4()` doesn't always have the whole payload in
memory to generate a real payload hash. this commit allows the user to
pass in a header like `x-amz-content-sha256` to provide their desired
payload hash

some services like s3 require this header, and may support other values
like s3's `UNSIGNED-PAYLOAD` and `STREAMING-AWS4-HMAC-SHA256-PAYLOAD`
with special semantics. servers use this header's value as the payload
hash during signature validation, so it must match what the client uses
to generate the signature

CURLOPT_AWS_SIGV4.3 now describes the content-sha256 interaction

Signed-off-by: Casey Bodley <cbodley@redhat.com>
Closes #9804