Add channel_id to SpendableOutputs event

Add channel_id to SpendableOutputs event
    This will make it possible to
    link between SpendableOuts and ChannelMonitor

    - change channel_id to option so we dont break upgrade
    - remove unused channel_id
    - document channel_id
    - extract channel id dynamically to pass test
    - use contains to check channel_id in test as the events are not ordered
    - update docs framing
    - specify ldk version channel_id will be introduced in

Co-authored-by: Elias Rohrer <dev@tnull.de>
Update lightning/src/events/mod.rs

Co-authored-by: Elias Rohrer <dev@tnull.de>