Move `MessageSendEvent` to `ln::msgs` from `events` #6145

Job	Run time
check_commits	5m 11s
benchmark	5m 13s
build (ubuntu-latest, stable)	1h 8m 12s
build-tx-sync (ubuntu-latest, stable)	4m 54s
build (ubuntu-latest, beta)	1h 5m 33s
build-tx-sync (ubuntu-latest, beta)	5m 4s
check_release	1m 57s
build (ubuntu-latest, 1.63.0)	1h 32m 8s
coverage	8m 30s
build-tx-sync (ubuntu-latest, 1.75.0)	5m 20s
build (windows-latest, stable)	1h 38m 34s
build-tx-sync (macos-latest, stable)	5m 58s
build (windows-latest, beta)	1h 38m 35s
fuzz	1h 38m 42s
build-tx-sync (macos-latest, beta)	5m 40s
incremental-mutants	1m 38s
build (macos-latest, stable)	1h 8m 30s
build (macos-latest, beta)	1h 16m 32s
linting	1m 5s
rustfmt	17s
build (macos-latest, 1.63.0)	1h 13m 41s
build-tx-sync (macos-latest, 1.75.0)	4m 44s
	13h 15m 58s

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