add content-sha256 header for presign when the signed_body_value is set to UNSIGNED-PAYLOAD

[TingDaoK]

Issue #, if available:

• When signing via query-params, there is no available way to sign content-sha256 header

Description of changes:

• When the signed_body_value is set to UNSIGNED-PAYLOAD, signer will add the content-sha256 header even signing via query-params.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.40%. Comparing base (877c029) to head (bb68463).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #248 +/- ## ======================================= Coverage 80.39% 80.40% ======================================= Files 33 33 Lines 6035 6038 +3 ======================================= + Hits 4852 4855 +3 Misses 1183 1183 ```

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