bin helpers: Various fixes to support --mode gamma and app_net

[sergiitk]

No framework code changed, just the bin scripts.

Tested with:

# Legacy
export PSM_EXTRA_FLAGS="--resource_suffix=legacy --server_xds_port=4343"
# mTLS
export PSM_EXTRA_FLAGS="--mode=secure --security=mtls --resource_suffix=mtls --server_xds_port=4545"
# AppNet
export PSM_EXTRA_FLAGS="--mode=app_net --resource_suffix=appnet --server_xds_port=4242"
# Gamma
export PSM_EXTRA_FLAGS="--mode=gamma --resource_suffix=gamma-http"
# Gamma GRPCRoute
export PSM_EXTRA_FLAGS="--mode=gamma --resource_suffix=gamma-grpc --gamma_route_kind=grpc"

Normal, AppNet setup

./run.sh bin/run_test_server.py \
  && ./run.sh bin/run_td_setup.py \
  && ./run.sh bin/run_td_setup.py --cmd=backends-add \
  && ./run.sh bin/run_test_client.py \
  && ./run.sh bin/run_ping_pong.py

CSM setup

./run.sh bin/run_test_server.py \
  && ./run.sh bin/run_test_client.py \
  && ./run.sh bin/run_ping_pong.py

[jarias-lfx]

/easycla

[XuanWang-Amos]

LGTM