Add a `BlindedMessagePath` constructor for external recipients #23519

Job	Run time
ext-test	5m 11s
check_release	3m 19s
tor-connect	5m 59s
linting	2m 36s
rustfmt	15s
fuzz	1h 30m 53s
fuzz_sanity	9m 41s
check_docs	8m 29s
build-nostd / build (self-hosted, 1.75.0)	21m 20s
build-sync / build (self-hosted, 1.75.0)	10m 24s
build-workspace / build (self-hosted, 1.75.0)	12m 14s
benchmark	17m 26s
build-features / build (self-hosted, 1.75.0)	35m 14s
build-bindings / build (self-hosted, 1.75.0)	19m 32s
build-cfg-flags / build (self-hosted, 1.75.0)	11m 20s
coverage	47m 31s
notify-failure	-1s
	5h 1m 23s

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