Factor out combine / finalize / extract PSBT helpers

Refactor the new CombinePSBT, FinalizePSBT, and FinalizeAndExtractPSBT
general-purpose functions out of the combinepsbt and finalizepsbt RPCs,
for use in the GUI code.

src/psbt.cpp

@@ -232,3 +232,52 @@ bool SignPSBTInput(const SigningProvider& provider, PartiallySignedTransaction&
 
     return sig_complete;
 }
+
+bool FinalizePSBT(PartiallySignedTransaction& psbtx)
+{
+    // Finalize input signatures -- in case we have partial signatures that add up to a complete
+    //   signature, but have not combined them yet (e.g. because the combiner that created this
+    //   PartiallySignedTransaction did not understand them), this will combine them into a final
+    //   script.
+    bool complete = true;
+    for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {
+        complete &= SignPSBTInput(DUMMY_SIGNING_PROVIDER, psbtx, i, SIGHASH_ALL);
+    }
+
+    return complete;
+}
+
+bool FinalizeAndExtractPSBT(PartiallySignedTransaction& psbtx, CMutableTransaction& result)
+{
+    // It's not safe to extract a PSBT that isn't finalized, and there's no easy way to check
+    //   whether a PSBT is finalized without finalizing it, so we just do this.
+    if (!FinalizePSBT(psbtx)) {
+        return false;
+    }
+
+    result = *psbtx.tx;
+    for (unsigned int i = 0; i < result.vin.size(); ++i) {
+        result.vin[i].scriptSig = psbtx.inputs[i].final_script_sig;
+        result.vin[i].scriptWitness = psbtx.inputs[i].final_script_witness;
+    }
+    return true;
+}
+
+bool CombinePSBTs(PartiallySignedTransaction& out, TransactionError& error, const std::vector<PartiallySignedTransaction>& psbtxs)
+{
+    out = psbtxs[0]; // Copy the first one
+
+    // Merge
+    for (auto it = std::next(psbtxs.begin()); it != psbtxs.end(); ++it) {
+        if (!out.Merge(*it)) {
+            error = TransactionError::PSBT_MISMATCH;
+            return false;
+        }
+    }
+    if (!out.IsSane()) {
+        error = TransactionError::INVALID_PSBT;
+        return false;
+    }
+
+    return true;
+}