Add ALTS support to interop stress client

[sagarsudo]

Tested using:

./client --server_addresses="google-c2p:///directpath-grpclb-with-pick-first-test-jz.googleapis.com" --test_cases="empty_unary:100" --use_alts=true --test_duration_secs=10
Total calls made: 54346384

[eshitachandwani]

Hi @sagarsudo, it seems you may want to include this flag in your application for testing purposes. Could you please clarify the reason for requesting this change in the gRPC-Go OSS repository?

[sagarsudo]

Just noticed a discrepancy in between interop/client and interop/stress/client, where the former has ALTS support but the latter does not.

[sagarsudo]

It would be good to have ALTS support in the stress client as it would make it easier to stress test against GCP services.