Add ping pong subtask to extproxy #14000

Job	Run time
towncrier_check	12s
check_if_release_branch	1s
changed_files	7s
build mirrord CLI	0s
lint	3m 49s
test_agent_image	57s
integration_tests	12m 14s
check-rust-docs	1m 1s
macos_tests	18m 35s
test_agent	3m 35s
test_cli_image	15m 28s
lint_markdown	0s
e2e (docker)	15m 14s
vscode_e2e_on_release_branch	0s
intellij_e2e_on_release_branch	0s
e2e (containerd)	12m 15s
ci	1s
	1h 23m 29s

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