[v2] Update checksum calculation and validation for vendored botocore

[aemous]

Issue #, if available:

• https://github.com/boto/botocore/pull/3277/files#

Description of changes:

• 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 conditionally generated (detailed in source PR)
• When the request_checksum_calculation config is set to when_required, a checksum will be generated conditionally (detailed in source PR):
• Deprecates existing customizations for MD5 checksums.
• A value set for requestValidationModeMember config by the user will be used by the SDK.
• If the requestValidationModeMember value is not set by the user, the requestValidationModeMember will conditionally be set to ENABLED (detailed in source PR)

Description of tests:

• Ran and passed all added botocore tests included in this PR.

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