Fixes#5287.
The PSBT spec is a bit vague when it comes to the WitnessUtxo field of
an input as it's not strictly required for witness inputs. Therefore
Electrum for example does not include the field.
We need to make the SegWit input check a bit more elaborate by looking
at the output script of the UTXO and also the redeem script in case it's
a nested SegWit spend.
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.
