Factor out Http auth parse logic, test it. (#4211) #11775

Job	Run time
build_details	3s
build_js	2m 4s
build_opam (4.14.x)	9m 20s
build_opam (5.2.x)	5m 8s
build_no_depopts	6m 33s
lezer_parse	53s
build_win32 (x64)	11m 21s
build_posix (debian_trixie, amd64)	11m 14s
build_posix (debian_bookworm, amd64)	11m 14s
build_posix (ubuntu_oracular, amd64)	11m 16s
build_posix (ubuntu_noble, amd64)	11m 16s
tree_sitter_parse	48s
build_posix (alpine, amd64)	11m 17s
run_tests (@citest)	11m 15s
update_doc	11m 18s
run_tests (@doctest)	8m 17s
run_tests (@mediatest)	11m 14s
fetch_s3_artifacts	1s
build_docker_minimal	1s
build_docker_alpine_minimal	1s
build_docker_alpine	1s
build_docker	1s
build_docker_release	1s
build_docker_release-minimal	1s
update_release	1s
	2h 14m 39s

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