xds: Enable dualstack flag

[arjan-bal]

The flag was enabled in master as part of #8126. This PR enabled only the xDS endpoints support, leaving the new pickfirst policy flag disabled.

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 disable this feature, set the environment variable GRPC_EXPERIMENTAL_XDS_DUALSTACK_ENDPOINTS to false (case insensitive).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.25%. Comparing base (bf0c885) to head (0620870).
Report is 1 commits behind head on v1.71.x.

Additional details and impacted files

@@             Coverage Diff             @@
##           v1.71.x    #8134      +/-   ##
===========================================
- Coverage    82.39%   82.25%   -0.15%     
===========================================
  Files          387      387              
  Lines        38954    38954              
===========================================
- Hits         32096    32041      -55     
- Misses        5554     5596      +42     
- Partials      1304     1317      +13     

see 19 files with indirect coverage changes

[easwars]

Why do we want to enable only one of the two flags?

[arjan-bal]

Why do we want to enable only one of the two flags?

All petiole policies use the new pickfirst for delegation. The only user-visible benefit of toggling the default pickfirst would be enabling Happy Eyeballs with a standalone pickfirst. After discussing with Doug and Mark, we decided it's acceptable to advertise xDS Dualstack support without changing the default pickfirst since known xDS pickfirst users do not want to use Dualstack.

Most gRPC users rely on the default pickfirst. By delaying its adoption as the default to a future release, we allow time to gather feedback from internal users and those using the master branch.