batteries: Allow wrapping the MessageRouter in ChannelManagerConstructor

[Synesso]

ChannelManagerConstructor hardcodes DefaultMessageRouter for onion message path finding, with no way to customize it. This mirrors the existing RouterWrapper hook (which covers the payment Router) with a MessageRouterWrapper for the onion MessageRouter.

Motivation

DefaultMessageRouter::find_path only returns paths whose first hop is a directly connected peer. Nodes that restrict outbound connections (e.g. for egress-policy reasons) cannot act on Event::ConnectionNeeded, so onion messages to unconnected destinations — BOLT 12 invoice requests in particular — buffer forever. Such deployments instead route onion messages through a trusted forwarding peer by prefixing the path with a connected node, which requires substituting the MessageRouter handed to the OnionMessenger (and ChannelManager).

Today the only options are vendoring a modified copy of ChannelManagerConstructor or reconstructing ChannelManager/OnionMessenger/PeerManager/BackgroundProcessor by hand: the msg_router field is private, concretely typed DefaultMessageRouter, and the OnionMessenger built in chain_sync_completed is not exposed. (We are currently carrying such a vendored copy.)

Design

Follows RouterWrapper as closely as possible:

• New MessageRouterWrapper interface with find_path and create_blinded_paths, each receiving the DefaultMessageRouter the constructor would otherwise have used, so implementations can delegate for any case they don't want to customize.
• Both constructors take a @Nullable MessageRouterWrapper parameter, immediately after router_wrapper. null preserves existing behavior exactly.
• The msg_router field becomes MessageRouter (it was only ever consumed via as_MessageRouter()).

Like the original RouterWrapper addition, this changes the public constructor signatures. Happy to add overloads preserving the old signatures instead if that's preferred.

[Synesso]

The CI failures here are pre-existing (upstream main is red for the same reasons). Opened #184 to repair the fixable ones — MSRV pins for unicode-ident/proc-macro2, retired macos-13 runners, and regenerated checked-in C# sources. The remaining bins-repo 404s (git.bitcoin.ninja) need maintainer-published binaries.