Default to CRC32 for Request Checksum Calculation

[jonathan343]

This PR makes the following updates to the botocore flexible checksum behavior:

General:

• Remove MD5 checksums and make CRC32 the default checksum algorithm.
• When the request_checksum_calculation config is set to when_supported (default value), a checksum will be generated whenever one of the following conditions is true:
  • An operation applies the httpchecksum trait and defines a requestAlgorithmMember.
  • An operation applies the httpchecksum trait and defines requestChecksumRequired: true.
• When the request_checksum_calculation config is set to when_required, a checksum will be generated only when:
  • An operation applies the httpchecksum trait and defines requestChecksumRequired: true.

S3 Customizations:

• Deprecates existing customizations for MD5 checksums.

~~"CreateMultipartUpload is modeled with a ChecksumAlgorithm member. This member signals to S3 the checksum algorithm that will be used for the related UploadPart operations. If this ChecksumAlgorithm differs from the SDK's default algorithm chosen for UploadPart operations, then S3 will return an error."~~

• ~~A set_default_multipart_checksum_algorithm handler was implemented to set ChecksumAlgorithm to CRC32 when it isn't set by the customer.~~

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Codecov Report

Attention: Patch coverage is 97.95918% with 1 line in your changes missing coverage. Please review.

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Files with missing lines	Patch %	Lines
botocore/utils.py	66.66%	1 Missing :warning:

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Additional details and impacted files

@@                   Coverage Diff                    @@
##             flexible-checksums-v2    #3277   +/-   ##
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[jonathan343]

This work is being tracked internally.