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 core#18027 [2/4]
https://github.com/bitcoin/bitcoin/pull/18027/commits/5dd0c03ffa3aeaa69d8a3a716f902f450d5eaaec

Depends on D11980

ninja all check-all

The new data will be exposed in the GUI in the next commit of this backport. For now we just check that the changes don't break existing tests.