interop: Introduce env var for xDS dualstack support and add xDS interop config

[arjan-bal]

Fixes: #7866

Why

This PR is required to start PSM interop testing for dualstack integration in Go. The interop test only tests the roundrobin LB policy which is already updated to handle endpoints in #7966.

Testing

Verified that the dualstack PSM interop tests pass when run using a client image built from this branch.

RELEASE NOTES:

• xds: LB policies will receive additional addresses for endpoints which are specified in the additional_addresses field in the Endpoint message. The additional addresses will be present in the ResolverState.Endpoints passed to UpdateClientConnState method of the balancers. To enable this feature, set the environment variable GRPC_EXPERIMENTAL_XDS_DUALSTACK_ENDPOINTS to true (case insensitive).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.31%. Comparing base (ad5cd32) to head (ef3d236).
Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8081      +/-   ##
==========================================
+ Coverage   82.10%   82.31%   +0.21%     
==========================================
  Files         387      387              
  Lines       39067    39064       -3     
==========================================
+ Hits        32076    32157      +81     
+ Misses       5662     5596      -66     
+ Partials     1329     1311      -18     

see 26 files with indirect coverage changes