I really like this PR, but it wound up being pretty nontrivial to merge.
The crux is that it pulls PSBT signing logic into scriptpubkey manager,
which is where it belongs, but for us this means reasoning about pegins
inside script/sign.cpp.
However, sign.cpp is part of libbitcoin_common, which does not include
anything for reasoning about PoW or RPC (lol) or anything heavy about
that. This means that some pegin validation had to remain split between
the wallet/rpc layer and sign.cpp. I added a new file script/pegins.cpp
which has the (one) method we actually need in sign.cpp.
Aside from that, this diff is very large but is mostly just moving our
code changes to wallet/psbtwallet.* into wallet/wallet.* where those
functions now live. As far as review, it's probably not worthwhile to
spend too much too much time on this since it's going to be change
again in #16528 and others. The test coverage is pretty extensive.
