Only set default checksum if request_checksum_calculation config is set to when_supported

[jonathan343]

AWS CLI v2 version of boto/s3transfer#329

Addresses: boto/s3transfer#327

This PR checks the request_checksum_calculation config on the S3 client before setting a default checksum for both non-MPU and MPU. If request_checksum_calculation is set to when_supported, the default checksum from botocore (currently CRC32) is applied. If request_checksum_calculation is set to when_required, no default checksum is applied.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (6323bb6) to head (e165279).
Report is 54 commits behind head on v2.

Additional details and impacted files

@@    Coverage Diff     @@
##   v2   #9238   +/-   ##
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