We add a new funding assembler and intent type that handle channel
funding through the use of a PSBT. The PsbtIntent is in itself a
simple state machine that can be stepped through the process of
assembling the required information for the funding output, verifying
a user supplied PSBT for correctness, accepting a fully signed PSBT
and then assembling the funding wire message.
In this commit, we introduce a series of new abstractions for channel
funding. The end goal is to enable uses cases that construct the funding
transaction externally, eventually handing the funding outpoint to lnd.
An example of such a use case includes channel factories and external
channel funding using a hardware wallet.
We also add a new chanfunding.Assembler meant to allow external channel
funding in contexts similar to how channel factories
can be constructed. With this channel funder, we'll only obtain the
channel point and funding output from it, as this alone is enough to
carry out a funding flow as normal.
