csdsclient: refactor code for adding support for v3 in the future

[Rainton]

This PR includes some code refactoring:

• Renamed csdspb to csdspb_v2 and changed Client into an interface{} for adding support for csdspb_v3 in the future

• Created ClientV2 which implements Client

• Moved ParseFlags() in the main to make code more reusable discussion with @alexburnos

• Extracted default values for flags out into constants discussion with @dschaller

• Added csdsv2_mock.go mock CSDS server generated by gomock for future tests

• Renamed flag -cloud_platform to -platform

Signed-off-by: Yutong Li yutongli@google.com
/cc @fuqianggao @alexburnos

[Rainton]

/cc @htuch @dschaller @dio @therealmitchconnors

[dio]

@alexburnos @fuqianggao please let me know if the recent changes resolve your concern. Thanks!