Trivial PeerManager cleanups#2249

Merged

TheBlueMatt merged 2 commits intolightningdevkit:mainfrom

TheBlueMatt:2023-04-less-pm-bounds

May 3, 2023

Commits on Apr 30, 2023

Move the `CustomMessageHandler` into the `MessageHandler` struct
TheBlueMatt
committed

Commits on May 2, 2023

Expose a trait impl'd for all `PeerManager` for use as a bound
TheBlueMatt
committed