Add ping pong subtask to extproxy #14021

Job	Run time
towncrier_check	7s
check_if_release_branch	1s
changed_files	11s
build mirrord CLI	0s
test_agent_image	7m 13s
integration_tests	12m 25s
check-rust-docs	1m 1s
lint	3m 33s
macos_tests	23m 12s
test_agent	3m 32s
test_cli_image	15m 33s
lint_markdown	0s
e2e (docker)	13m 35s
intellij_e2e_on_release_branch	0s
vscode_e2e_on_release_branch	0s
e2e (containerd)	13m 2s
ci	1s
	1h 33m 26s

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