do not require checksum algorithm when checksum is set

[sbiscigl]

Description of changes:

There is an issue where if you specify a checksum but not the checksum algorithm the functions ChecksumAlgorithmHasBeenSet and GetChecksumAlgorithmName would not return the expected value. This issue especially shows its head when you are using s3express and do something like

const auto client = Aws::MakeUnique<S3Client>("test");
auto request = PutObjectRequest().WithBucket(BUCKET_NAME)
    .WithKey("test-key")
    .WithChecksumSHA256(HashingUtils::Base64Encode(HashingUtils::CalculateSHA256("somebody")));

std::shared_ptr<IOStream> body = Aws::MakeShared<StringStream>("Test",
  "somebody",
  std::ios_base::in | std::ios_base::binary);
request.SetBody(body);
const auto response = client->PutObject(request);

we use the logic in ChecksumAlgorithmHasBeenSet and GetChecksumAlgorithmName to determine if we need to default to crc32 as s3express does not support md5.

This changes to call SetChecksumAlgorithm when setting a checksum so that the implicit requirement of setting the checksum algorithm is no longer required.

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• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
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