Rework ChannelManager::funding_transaction_signed #21173

Job	Run time
ext-test	5m 29s
benchmark	16m 11s
build (self-hosted, beta)	1h 59m 48s
build (self-hosted, 1.75.0)	2h 1m 20s
build (self-hosted, stable)	1h 39m 44s
build (macos-latest, stable)	2h 11m 34s
build (windows-latest, stable)	2h 21m 1s
build (macos-latest, 1.75.0)	1h 44m 50s
build-tx-sync (ubuntu-latest, stable)	4m 53s
build-tx-sync (ubuntu-latest, beta)	4m 46s
check_release	3m 1s
build-tx-sync (ubuntu-latest, 1.75.0)	5m 51s
build-tx-sync (macos-latest, beta)	5m 20s
build-tx-sync (macos-latest, stable)	5m 6s
fuzz	1h 16m 35s
rustfmt	16s
build-tx-sync (macos-latest, 1.75.0)	10m 0s
linting	1m 5s
check_docs	3m 44s
coverage	46m 28s
	15h 7m 2s

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