Add constructor with s3Client factory to s3-encryption

[sbiscigl]

Issue #, if available:

#3388

Description of changes:

Right now it is impossible for customers to create a S3 encryption client using a S3ClientConfiguration object. Instead of adding more constructors to match each S3Client constructor, add factory method so user can bring their own S3Client. i.e.

#include <aws/core/Aws.h>
#include <aws/s3-encryption/S3EncryptionClient.h>

using namespace Aws;
using namespace Aws::S3Encryption;
using namespace Aws::S3;
using namespace Aws::Utils;

namespace {
const char* LOG_TAG = "test-app";
}

auto main() -> int {
  SDKOptions options{};
  options.loggingOptions.logLevel = Logging::LogLevel::Debug;
  InitAPI(options);
  {
    CryptoBuffer symmetric_key{ByteBuffer{"symmetric_key"}};
    const auto simple_materials =
        Aws::MakeShared<Materials::SimpleEncryptionMaterialsWithGCMAAD>(
            LOG_TAG, symmetric_key);
    CryptoConfigurationV2 configuration_v2{simple_materials};
    S3ClientConfiguration client_configuration{};
    client_configuration.useVirtualAddressing = true;
    S3EncryptionClientV2 client_v2{
        configuration_v2, [&client_configuration]() -> UniquePtr<S3Client> {
          return Aws::MakeUnique<S3Client>(LOG_TAG, client_configuration);
        }};
  }
  ShutdownAPI(options);
  return 0;
}

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.

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