In FillPSBT, optionally report the number of inputs we successfully
signed, as an out parameter. If "sign" is false, instead report the
number of inputs for which GetSigningProvider does not return nullptr.
(This is a potentially overbroad estimate of inputs we could sign.)
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/18027 | core#18027]] [2/4]
https://github.com/bitcoin/bitcoin/pull/18027/commits/5dd0c03ffa3aeaa69d8a3a716f902f450d5eaaec