enhancement: flexible checksums

[stobrien89]

Description of changes:
Adds default checksum calculation, deprecates the AddContentMD5 config option as S3 plans to deprecate MD5 checksums soon, adds two S3 client config options: request_checksum_calculation and response_checksum_validation with acceptable values for both being either when_supported or when_required.

request_checksum_calculation will default to when_supported, which means if an operation supports checksum calculation (indicated via modeling), the SDK will calculate the checksum and add the appropriate header. If it is set to when_required, the SDK will only compute and send a checksum when the operation requires an httpRequestChecksum.

response_checksum_validation also defaults to when_supported, which means if an operation models response algorithms, an attempt to validate a response checksum will be made. when_required will only validate response checksums when response checksum are modeled AND ChecksumMode is set to enabled on the command.

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[yenfryherrerafeliz]

Overall looks good. Just some minor changes. I also did not see a test for when generating a presigned url with PUT operations and the checksum here should be omitted?

[yenfryherrerafeliz]

It looks great. Just two minor suggestion that are not required to address.

[yenfryherrerafeliz]

nit: I think this could be simplified to:

$supportedAlgorithms =  array_map('strtolower', $checksumMember['enum'] ?? []);

[yenfryherrerafeliz]

There is another place that validates if the checksum is valid or not. For example here. I know one validates if is valid based on what the operation supports and the other one validates if is supported by the SDK in specific. Would not be worth to make that more clear here?, like state why it is not supported.
For example:

  throw new InvalidArgumentException(
      "Invalid checksum requested: {$requestedAlgorithm}."
      . "  Valid algorithms supported by this runtime are {$validAlgorithms}."
  );