Add a new class to `@aws-cdk/aws-certificatemanager` called
`DNSValidatedCertificate`.  This class generates a certificate request
using AWS Certificate Manager and auto-validates the request using the
provided DNS "magic cookie" records.  The user need only supply the
Domain Name of the certificate and a Route 53 Hosted Zone.  A
CloudFormation Custom Resource is used along with the supplied Lambda
function to perform the operation.

Eliminate all third-party dependencies in dns_validated_certificate
Custom Resource function.

…ted_certificate