fix unknown checksum handling

[DmitriyMusatkin]

Issue #, if available:

Description of changes:
Before this pr sending default request with a body that had any header starting with "x-amz-checksum-" that was not a valid checksum would fail with a generic error about failing to asign the body. Ex. completeMPU call going through default that had a body and x-amz-checksum-type header. Note: this is manual call to completeMPU though default meta request and usual flow is not affected.

2 changes made in this pr to mitigate the issue:

• dont treat headers like x-amz-checksum-type, x-amz-checksum-mode and x-amz-checksum-algorithm and unknown checksum type
• narrow down the cases where checksum is recomputed for default case. i.e. if req does not need checksum and there is no verification callback then do not compute it. checksum is never recomputed for completeMPU.

Note: there is still a weird case that if checksum is provided for default with body and there is a verification callback, then we recompute checksum. on one side it makes sense to recompute on the other it does not. so leaving as is for now

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

[TingDaoK]

LGTM

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.98%. Comparing base (10be224) to head (6f63ac3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #633      +/-   ##
==========================================
+ Coverage   88.96%   88.98%   +0.01%     
==========================================
  Files          23       23              
  Lines        7759     7769      +10     
==========================================
+ Hits         6903     6913      +10     
  Misses        856      856              

Files with missing lines	Coverage Δ
source/s3_auto_ranged_put.c	91.14% <100.00%> (+0.02%)	⬆️
source/s3_checksum_context.c	88.37% <100.00%> (+0.13%)	⬆️
source/s3_checksums.c	83.95% <100.00%> (ø)
source/s3_default_meta_request.c	94.31% <100.00%> (+0.06%)	⬆️
source/s3_request_messages.c	72.67% <100.00%> (ø)
source/s3_util.c	95.21% <100.00%> (+0.06%)	⬆️

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