Create ln-dlc-node sub-crate

[luckysori]

The purpose of this library crate is to provide the API that an application such as the 10101 app would want to use to be able to implement the features described in #34.

Even though it is a library, we need to remember that at this stage we only expect two consumers: app and coordinator. Thus, let's avoid premature generalisations.

Observations

• Eventually we should be able to move most of the code we write to p2pderivates/rust-dlc, but that requires more coordination than what we can afford right now.
• Depends on our forks of rust-dlc and rust-lightning.