The current v0 extractor code only works for the happy path, it does not
do checks on the input PSBT as specified by the bip.

Add a TODO and also copy the relevant text from bip-174.

This code was just cut'n'paste and isn't currently being built. Just
delete it and we will start from scratch.

Add two methods to the `InputBuilder` to add funding utxos. Note that it
would really be better to use the type system to enforce existence of
the funding utxo but doing so would take us away from `Input` being a
1-1 representation of the bip.

Add types for the two new roles. Feature gate the `Finalizer` on
"miniscript". Because another entity may be the Finalizer do not feature
gate the `Extractor` oven thought transaction extraction is only
possible for a finalized PSBT.