Display total amount of external inputs when reliable

While warning for the presence of external inputs remains necessary
(certain attacks are possible), we can show additional info when
the amount of external inputs is committed to (that is, when the
sighash flag we're signing with does not have ANYONECANPAY, and
therefore the signature commits to all inputs).

In that case, we can show:
- The total amount of external inputs
- The net spend (or receive) from/to the wallet policy, similarly
  to the transaction summary shown with certain sighash flags.