This adds a new fn to ChannelKeys which is called when we generte
a new remote commitment transaction for signing. While it may be
theoretically possible to unwind state updates by disconnecting and
reconnecting as well as making appropriate state machine changes,
the effort required to get it correct likely outweighs the UX cost
of "preflighting" the requests to hardwre wallets.
Instead of having in-memory access to the list of private keys
associated with a channel, we should have a generic API which
allows us to request signing, allowing the user to store private
keys any way they like.
The first step is the (rather mechanical) process of templating
the entire tree of ChannelManager -> Channel impls by the
key-providing type. In a later commit we should expose only public
keys where possible.
