xds: Expose Dualstack support using an environment variable

[arjan-bal]

Presently not all LB policies handle endpoints, some still read resolver.Addresses. These LB policies will need to be changed incrementally to handle endpoints correctly. The clusterresolver will be able to produce multiple addresses for each backend after #7858 is submitted, however that feature is behind a variable that can't be controlled via an env variable to hide the Dualstack support while its under development.

We should make the feature flag configurable with an environment variable once LB polices are updated.