Move CounterpartyForwardingInfo from channel to channelmanager

Move CounterpartyForwardingInfo from channel to channelmanager

CounterpartyForwardingInfo is public (previously exposed with a
`pub use`), and used inside of ChannelCounterparty in
channelmanager.rs. However, it is defined in channel.rs, away from
where it is used.

This would be fine, except that the bindings generator is somewhat
confused by this - it doesn't currently support interpreting
`pub use` as a struct to expose, instead ignoring it.

Fixes https://github.com/lightningdevkit/ldk-garbagecollected/issues/44