Implementing checksum validation for single-part and multipart downloads

[pulimsr]

Description of changes: Implemented per-part checksum validation for downloads in TransferManager. Added validateChecksums flag (default: true) to TransferManagerConfiguration. Downloads now validate checksums for both single-part (via file read after download) and multipart transfers (per-part validation). Created GetChecksumFromResult() helper to reduce code duplication. Downloads fail immediately on checksum mismatch.

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• [x] Did a review by yourself.
• [x] Added proper tests to cover this PR. (If tests are not applicable, explain.)
• [x] Checked if this PR is a breaking (APIs have been changed) change.
• [x] Checked if this PR will not introduce cross-platform inconsistent behavior.
• [x] Checked if this PR would require a ReadMe/Wiki update.

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