xds, pickfirst: Enable additional addresses in xDS, set new pick_first as default

[arjan-bal]

Tested

• Ran all internal tests (global TAP) with the flags turned on, verified none of the failures were related.
• PSM interop Dual Stack tests passing since 14th Feb.

RELEASE NOTES:

• pickfirst: The new pick first LB policy is made the default. The new LB policy implements the Happy Eyeballs algorithm (see A61) to connect to multiple addresses concurrently. To disable the new policy set the environment variable GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST to false (case insensitive).
• 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 diable 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 82.26%. Comparing base (dbf92b4) to head (aa373c9).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8126      +/-   ##
==========================================
+ Coverage   82.18%   82.26%   +0.08%     
==========================================
  Files         387      387              
  Lines       38947    38947              
==========================================
+ Hits        32007    32041      +34     
+ Misses       5613     5586      -27     
+ Partials     1327     1320       -7     

Files with missing lines	Coverage Δ
internal/envconfig/envconfig.go	100.00% <ø> (ø)

... and 18 files with indirect coverage changes