In order to help external components to query the custom records of a
channel we need to expose the remote peer pub key. We could look-up
custom records based on the funding outpoint, but that relation is
established when receiving the ChannelReady message. The external
components may query the AuxChanState before that message is received,
so let's make sure the peer pub key is also available.
In this commit, we add a new AuxLeafStore which can be used to dynamically fetch the latest aux leaves for a given state. This is useful for custom channel types that will store some extra information in the form of a custom blob, then will use that information to derive the new leaf tapscript leaves that may be attached to reach state.
