GetPropertiesOfCertificates needs to be resilient and not fail with JSON error whenever the x5t property is missing from the response

[ahsonkhan]

We cannot always assume that an x5t property will be present within the JSON response:

azure-sdk-for-cpp/sdk/keyvault/azure-security-keyvault-certificates/src/certificate_serializers.cpp

Lines 42 to 43 in bef4201

	// x5t
	properties.X509Thumbprint = Base64Url::Base64UrlDecode(jsonResponse[X5tName].get<std::string>());

This needs to be within a check:

    if (certificate.contains(X5tName))
    {
      properties.X509Thumbprint
          = Base64Url::Base64UrlDecode(certificate[X5tName].get<std::string>());
    }

azure-sdk-for-cpp/sdk/core/azure-core/inc/azure/core/internal/json/json.hpp

Lines 25662 to 25668 in bef4201

	// const operator[] only works for objects
	if (_azure_JSON_HEDLEY_LIKELY(is_object()))
	{
	auto it = m_data.m_value.object->find(key);
	_azure_JSON_ASSERT(it != m_data.m_value.object->end());
	return it->second;
	}

Otherwise, the JSON library's [] operator behavior is undefined (and in debug the test/app crashes with an assertion failure and abort).

C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.41.34120\include\xtree(182) : Assertion failed: cannot dereference end map/set iterator

Steps to repo:
Right after a certificate is first created (and it is still pending), if the customer calls GetPropertiesOfCertificates with options.IncludePending = true;, the application could fail due to the x5t property missing.

      auto keyVaultUrl = "https://<keyvault-name>.vault.azure.net/";
      auto cred = std::make_shared<AzureCliCredential>();
      CertificateClient client(keyVaultUrl, cred);

      // Create a certificate
      std::cout << "Creating certificate atest-cert99" << std::endl;
      std::string certificateName = "atest-cert99";

      CertificateCreateOptions options;
      // create a lifetime action
      LifetimeAction action;
      action.LifetimePercentage = 80;
      action.Action = CertificatePolicyAction::AutoRenew;

      options.Properties.Enabled = true;
      options.Policy.Subject = "CN=sample1";
      options.Policy.ValidityInMonths = 12;
      options.Policy.Enabled = true;
      options.Policy.ContentType = CertificateContentType::Pkcs12;
      options.Policy.IssuerName = "Self";
      options.Policy.LifetimeActions.emplace_back(action);

      options.Properties.Name = certificateName;
      client.StartCreateCertificate(certificateName, options);

      // Fetch list of certificates
      // Observe that this will fail
      GetPropertiesOfCertificatesOptions options;
      options.IncludePending = true;
      std::cout << "Certificates in the key vault (includePending = true):" << std::endl;
      for (auto cert = client.GetPropertiesOfCertificates(options); cert.HasPage();
           cert.MoveToNextPage())
      {
        std::cout << "Found " << cert.Items.size() << " certificates." << std::endl;
        for (auto item : cert.Items)
        {
          std::cout <<  item.Name << std::endl;
        }
      }