Add PinHandlingNetworkAPI to support bridging message pins

Summary

Currently bridgev2 ignores m.room.pinned_events state changes. They fall through to the default case in handleMatrixEvent and are silently dropped. This means network connectors have no way to handle pin/unpin actions coming from Matrix.

I'm working on adding message pinning support to mautrix-whatsapp (mautrix/whatsapp#905). The WhatsApp to Matrix direction is already working by updating the room's m.room.pinned_events state directly. However, the Matrix to WhatsApp direction requires bridgev2 to dispatch pinned events state changes to the connector.

What's needed

A new PinHandlingNetworkAPI optional interface, following the same pattern as RoomNameHandlingNetworkAPI, TagHandlingNetworkAPI, and other existing handler interfaces. This would allow connectors to receive pin/unpin actions from Matrix with the added and removed event IDs, so they can be forwarded to the remote network.

I'm happy to implement this if the approach sounds good.