Make S3 checksum settings configurable via JSON config

[serdarozerr]

Previously, the three S3 checksum calculation flags (RequestChecksumCalculationEnabled, ResponseChecksumCalculationEnabled, UploaderRequestChecksumCalculationEnabled) were hardcoded struct fields with no JSON binding, meaning they could not be set from the config file.

This change adds json struct tags to all three fields, exposing them as optional config parameters:

request_checksum_calculation_enabled (default: true)
response_checksum_calculation_enabled (default: true)
uploader_request_checksum_calculation_enabled (default: true)