[interop client] provide a flag to set google-c2p resolver universe domain

[apolcyn]

Documentation PR: grpc/grpc#38933

RELEASE NOTES: N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.43%. Comparing base (8c080da) to head (f9c06c8).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8145      +/-   ##
==========================================
+ Coverage   82.39%   82.43%   +0.04%     
==========================================
  Files         389      389              
  Lines       39125    39125              
==========================================
+ Hits        32238    32254      +16     
+ Misses       5568     5556      -12     
+ Partials     1319     1315       -4     

see 18 files with indirect coverage changes

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[arjan-bal]

LGTM, I think we should wait for grpc/grpc#38933 to be merged before merging this to ensure the flag name and behaviour are finalized.

[easwars]

@apolcyn : Are we still waiting on grpc/grpc#38933, or should we close this?

[apolcyn]

@apolcyn : Are we still waiting on grpc/grpc#38933, or should we close this?

Lost track of grpc/grpc#38933, we still want this though