Add (Inbound/Outbound)V2Channels & DualFundingChannelContext wrapper

In preparation for upcoming commits introducing V2 channel establishment,
we introduce 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.

Author: dunxen