Add ping pong subtask to extproxy #14020

Job	Run time
towncrier_check	8s
check_if_release_branch	1s
changed_files	6s
build mirrord CLI	0s
check-rust-docs	57s
lint	2m 47s
test_agent	2m 37s
integration_tests	3m 6s
macos_tests	2m 49s
test_agent_image	2m 39s
test_cli_image	2m 41s
lint_markdown	0s
vscode_e2e_on_release_branch	1s
intellij_e2e_on_release_branch	1s
e2e	1s
ci	1s
	17m 55s

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