Implement V2 channel establishment

[dunxen]

This PR aims to do the following:

• [ ] Introduce V2 prefunded channel types with state specific to dual-funding.
• [ ] Add functionality to ChannelManager to create and accept dual-funded channels.
  • [ ] Can set config contribute_to_dual_funded_channels if the user would like to contribute inputs to incoming dual-funded channels. In this case, these channels will need to be accepted manually, indicated by a new OpenChannelV2Request event so that inputs can be provided. If the above flag is false (default), then no inputs will be contributed and the channel is accepted on the user's behalf gated by all the commonly enforced rules. Other policies and functionality to automate funding of incoming channels may be introduced in the future.
• [ ] Introduce dual-funded channel feature bits