Add (Inbound/Outbound)V2Channels & DualFundingChannelContext wrapper

In preparation for upcoming commits introducing V2 channel establishment,
we add the V2 equivalents of the prefunded channel types.
They have a new distinct context struct, `DualFundingChannelContext`,
which includes fields specific to prefunded V2 channels. This new context
wraps the common `ChannelContext` which holds state that is common to all
channel types.