Rework ChannelManager::funding_transaction_signed #21172

Job	Run time
ext-test	7m 50s
build (self-hosted, beta)	15m 1s
build (self-hosted, 1.75.0)	16m 9s
build (windows-latest, stable)	26m 42s
benchmark	16m 21s
check_release	3m 18s
linting	58s
rustfmt	13s
fuzz	48m 41s
build (macos-latest, 1.75.0)	19m 52s
build (self-hosted, stable)	9m 27s
build-tx-sync (ubuntu-latest, stable)	4m 47s
build-tx-sync (ubuntu-latest, beta)	4m 53s
build-tx-sync (macos-latest, beta)	6m 35s
build-tx-sync (macos-latest, 1.75.0)	7m 9s
build-tx-sync (macos-latest, stable)	5m 57s
build-tx-sync (ubuntu-latest, 1.75.0)	5m 49s
build (macos-latest, stable)	16m 43s
check_docs	3m 52s
coverage	1s
	3h 40m 18s

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