add if match statement to download

[sbiscigl]

Description of changes:

Adds a etag if match statement to the ranged get functionality of the transfer manager. Will error in the event a file is changed mid operation.

i.e.

#include <aws/core/Aws.h>
#include <aws/core/utils/threading/PooledThreadExecutor.h>
#include <aws/transfer/TransferManager.h>

using namespace Aws;
using namespace Aws::S3;
using namespace Aws::Transfer;
using namespace Aws::Utils;
using namespace Aws::Utils::Threading;
namespace {
const char * LOG_TAG = "test-app";
const char * BUCKET_NAME = "somebucket";
const char * KEY_NAME = "somekey";
}

auto main() -> int {
  SDKOptions options{};
  options.loggingOptions.logLevel = Logging::LogLevel::Debug;
  InitAPI(options);
  {
    const auto executor = Aws::MakeUnique<PooledThreadExecutor>(LOG_TAG, std::thread::hardware_concurrency());
    TransferManagerConfiguration configuration{executor.get()};
    configuration.s3Client = Aws::MakeShared<S3Client>(LOG_TAG);
    configuration.transferInitiatedCallback = [](const TransferManager*, const std::shared_ptr<const TransferHandle>&) {
      std::cout << "started transfer\n";
    };

    configuration.errorCallback = [](const TransferManager*, const std::shared_ptr<const TransferHandle>&, const Aws::Client::AWSError<Aws::S3::S3Errors>& error) {
      std::cout << "error: " << error.GetMessage() << "\n";
    };

    configuration.transferStatusUpdatedCallback = [](const TransferManager*, const std::shared_ptr<const TransferHandle>& handle) {
      if (handle->GetStatus() == TransferStatus::COMPLETED) {
        std::cout << "completed transfer\n";
      } else if (handle->GetStatus() == TransferStatus::FAILED) {
        std::cout << "failed\n";
      }
    };

    const auto transferManager = TransferManager::Create(configuration);

    const auto handle = transferManager->DownloadFile(BUCKET_NAME, KEY_NAME, KEY_NAME);

    transferManager->WaitUntilAllFinished();
  }
  ShutdownAPI(options);
  return 0;
}

will result in the output

error: Unable to parse ExceptionName: PreconditionFailed Message: At least one of the pre-conditions you specified did not hold
failed
finished transfer

when run when a objects etag changes and objects are not versioned. No file will be downloaded.

Check all that applies:

• 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.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

Not adding a test as it requires a race condition i cannot force in sdk code

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