Implement combine function for CRC32

[hssyoo]

Ports this reference Rust implementation: https://lenain.info/crc32-combine-in-rust/

This PR implements a function to combine 2 CRC32-computed values (ie CRC32(A) + CRC32(B)) to produce a single CRC32 value that is equal to CRC32(A+B). For example, the combination CRC32(foo) + CRC32(bar) would be equal to CRC32(foobar).

The purpose of this function is to enable parallel checksum calculation when uploading/downloading an object across multiple parts. Using a single running checksum object that all parts must update would cause head-of-line blocking, forcing already uploaded/downloaded parts to persist in memory while waiting to update the checksum object. Instead, the combine function allows each part to have its own checksum object, which are then serially combined at the end to produce a single full object checksum.

combine_crc32 takes as input CRC32 integer values that are available in botocore's Crc32Checksum class: https://github.com/boto/botocore/blob/d3ade36d635a2b6a89229b199234afc52f9bcf55/botocore/httpchecksum.py#L82, which will be exposed in followup PRs.

Example usage of this function can be seen in a draft PR: aws/aws-cli#9660

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Codecov Report

❌ Patch coverage is 97.67442% with 1 line in your changes missing coverage. Please review.
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Files with missing lines	Patch %	Lines
s3transfer/checksums.py	97.67%	1 Missing ⚠️
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