Instead of using CombineSignatures to create the final scriptSig or
scriptWitness of an input, use ProduceSignature itself.

To allow for ProduceSignature to place signatures, pubkeys, and scripts
that it does not know about, we pass down the SignatureData to SignStep
which pulls out the information that it needs from the SignatureData.

This is a partial backport of Core PR13425 : https://github.com/bitcoin/bitcoin/pull/13425/commits/ed94c8b556dbbfb62452eaefd9ee7841df09777a

Depends on D4205 and D4210