title: How to get PrivateIPAddress of VPC Endpoint in CDK?
body: <p>I need create a VPC Endpoint and an ALB to target the VPC Endpoint in CDK. </p>
  
  <p>I found InterfaceVpcEndpoint can return vpcEndpointNetworkInterfaceIds attribute. So it seems the missing part is how to get private IP address from these ENI IDs in a CDK way. </p>
  
  <p>I found CDK has a custom-resource package, its example shows I can use AwsCustomResource to call an AWS API (EC2/DescribeNetworkInterfaces) to get the IP Address. </p>
  
  <p>I tried write a custom resource like below:</p>
  
  <pre class="lang-py prettyprint-override"><code>eni = AwsCustomResource(
          self, 'DescribeNetworkInterfaces',
          on_create=custom_resources.AwsSdkCall(
              service='ec2',
              action='describeNetworkInterfaces',
              parameters= {
                  'NetworkInterfaceId.N': [eni_id]
              },
              physical_resource_id=str(time.time())
          )
      )
  ip = eni.get_data('NetworkInterfaces.0.PrivateIpAddress')
  </code></pre>
  
  <p>and pass ip into elbv2.IPTarget.</p>
  
  <p>But it seems I missed something, so it complains it needs a scalar not reference?</p>
  
  <pre><code>(.env) ➜  base-stack (master) ✔ cdk synth base --no-staging &gt; template.yaml
  jsii.errors.JavaScriptError: 
    Error: Expected Scalar, got {"$jsii.byref":"@aws-cdk/core.Reference@10015"}
        at Object.deserialize (/Volumes/DATA/ci/aws/base-stack/.env/lib/python3.7/site-packages/jsii/_embedded/jsii/jsii-runtime.js:12047:23)
        at Kernel._toSandbox (/Volumes/DATA/ci/aws/base-stack/.env/lib/python3.7/site-packages/jsii/_embedded/jsii/jsii-runtime.js:7031:61)
        at /Volumes/DATA/ci/aws/base-stack/.env/lib/python3.7/site-packages/jsii/_embedded/jsii/jsii-runtime.js:7084:33
        at Array.map (&lt;anonymous&gt;)
        at Kernel._boxUnboxParameters (/Volumes/DATA/ci/aws/base-stack/.env/lib/python3.7/site-packages/jsii/_embedded/jsii/jsii-runtime.js:7084:19)
        at Kernel
  ....
  </code></pre>
  
  <p>Thanks!</p>