Service specific presign

[TingDaoK]

Issue #, if available:

• Revert the previous heck we did. add content-sha256 header for presign when the signed_body_value is set to UNSIGNED-PAYLOAD #248
• Now, force unsigned payload for vpc-lattice-svcs and add the header as they expected.
• IT IS A TEMPORARY WORKAROUND, SHOULD BE REMOVED IN THE NEW FUTURE

Description of changes:

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

Attention: Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 80.36%. Comparing base (1de5445) to head (5c5dcd1).

Files with missing lines	Patch %	Lines
source/aws_signing.c	60.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #249      +/-   ##
==========================================
- Coverage   80.40%   80.36%   -0.05%     
==========================================
  Files          33       33              
  Lines        6038     6044       +6     
==========================================
+ Hits         4855     4857       +2     
- Misses       1183     1187       +4     

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