Support trailing checksum with no signing

[TingDaoK]

Issue #, if available:

• When you do trialing checksum, apparently, you need to have the x-amz-content-sha256 header to be set to the special value even if you are not signing the request.

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 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.67%. Comparing base (5d8d420) to head (5c9e4af).

Files with missing lines	Patch %	Lines
source/s3_meta_request.c	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #459      +/-   ##
==========================================
+ Coverage   89.63%   89.67%   +0.03%     
==========================================
  Files          20       20              
  Lines        6137     6141       +4     
==========================================
+ Hits         5501     5507       +6     
+ Misses        636      634       -2     

Files with missing lines	Coverage Δ
source/s3_client.c	90.09% <100.00%> (ø)
source/s3_meta_request.c	92.05% <86.66%> (+0.32%)	⬆️

... and 1 file with indirect coverage changes