Support proxying client requests through direct and tunneling HTTP/HTTPS proxies #1761

Job	Run time
build-test-cohttp-bench (ubuntu-latest, 5, *.opam )	2m 44s
build-test-cohttp-async (ubuntu-latest, 5, *.opam !cohttp-mirage.opam !cohttp-curl-lwt.opam !coht...	3m 54s
build-test-default (ubuntu-latest, 5.2, *.opam !cohttp-eio.opam !cohttp-curl-async.opam !cohttp-a...	5m 0s
build-test-cohttp-async (macos-latest, 5, *.opam !cohttp-mirage.opam !cohttp-curl-lwt.opam !cohtt...	1m 54s
build-test-cohttp-eio (ubuntu-latest, 5, http.opam cohttp.opam cohttp-eio.opam )	2m 52s
build-test-default (macos-latest, 5.2, *.opam !cohttp-eio.opam !cohttp-curl-async.opam !cohttp-as...	5m 41s
build-test-cohttp-eio (macos-latest, 5, http.opam cohttp.opam cohttp-eio.opam )	4m 51s
	26m 56s

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