Expose `channel_id` / `counterparty_node_id` in `BumpTransaction` event
A client node might choose not to handle `Event::BumptTransaction`
events and leave bumping / Anchor output spending to a trusted
counterparty.
However, `Event::BumptTransaction` currently doesn't offer any clear
indication what channel and/or counterparty it is referring to. In order
to allow filtering these events, we here expose the `channel_id` and
`counterparty_node_id` fields.