Add ping pong subtask to extproxy (#3039) #14022

Job	Run time
towncrier_check	7s
check_if_release_branch	1s
changed_files	5s
build mirrord CLI	0s
check-rust-docs	1m 6s
lint	3m 38s
test_agent_image	6m 46s
test_agent	3m 38s
integration_tests	13m 40s
macos_tests	24m 20s
test_cli_image	15m 25s
lint_markdown	0s
e2e (docker)	11m 7s
intellij_e2e_on_release_branch	0s
vscode_e2e_on_release_branch	0s
e2e (containerd)	11m 50s
ci	1s
	1h 31m 44s

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