Support async signing of splice shared input #23276

Job	Run time
tor-connect	7m 16s
build-bindings / build (self-hosted, 1.75.0)	18m 35s
fuzz_sanity	11m 9s
build-nostd / build (self-hosted, 1.75.0)	21m 34s
rustfmt	16s
check_docs	5m 27s
build-features / build (self-hosted, 1.75.0)	35m 39s
benchmark	17m 26s
ext-test	7m 45s
fuzz	43m 50s
build-workspace / build (self-hosted, 1.75.0)	13m 51s
build-cfg-flags / build (self-hosted, 1.75.0)	17m 39s
build-sync / build (self-hosted, 1.75.0)	24m 46s
linting	2m 32s
check_release	3m 5s
coverage	18m 39s
notify-failure	0s
	4h 9m 29s

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