feat(client): add `proxy::Tunnel` legacy util #447

Job	Run time
Check Style	12s
semver	20s
features	47s
Check MSRV (1.63)	2m 9s
Test with Miri	45s
Test stable on ubuntu-latest	37s
Test stable on windows-latest	1m 15s
Test stable on macos-latest	33s
Test beta on ubuntu-latest	38s
Test beta on windows-latest	1m 6s
Test beta on macos-latest	40s
Test nightly on ubuntu-latest	34s
Test nightly on windows-latest	1m 12s
Test nightly on macos-latest	35s
Build docs	12s
CI is green	0s
	11m 35s

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