Add physical ID to resource equivalence

[otaviomacedo]

Modify the digest function to include physical ID, if it's defined:

d(resource) = hash(type + physicalId)                       , if physicalId is defined
            = hash(type + properties + dependencies.map(d)) , otherwise

Note: The information about which field is the physical ID can be obtained from awscdk-service-spec, using the CloudFormationRegistryResource.primaryIdentifier attribute.