Replace PSBT.tx with PSBT::GetUnsignedTx and PSBT::GetUniqueID

The global unsigned tx is decomposed into separate fields inside of
PSBT, which mirrors what PSBTv2 will do. However, we still need to get
the global unsigned tx so PSBT::GetUnsignedTx is introduced to do that.
In order to also have a stable unique ID, we also introduce
PSBT::GetUniqueID to replace uses of PSBT.tx.GetHash().

src/node/psbt.cpp

@@ -18,6 +18,13 @@ PSBTAnalysis AnalyzePSBT(PartiallySignedTransaction psbtx)
     // Go through each input and build status
     PSBTAnalysis result;
 
+    std::optional<CMutableTransaction> unsigned_tx = psbtx.GetUnsignedTx();
+    if (!unsigned_tx) {
+        result.SetInvalid("PSBT cannot be made into a valid transaction");
+        return result;
+    }
+    CMutableTransaction& mtx = *unsigned_tx;
+
     bool calc_fee = true;
 
     CAmount in_amt = 0;
@@ -116,7 +123,6 @@ PSBTAnalysis AnalyzePSBT(PartiallySignedTransaction psbtx)
         result.fee = fee;
 
         // Estimate the size
-        CMutableTransaction mtx(*psbtx.tx);
         CCoinsViewCache view{&CoinsViewEmpty::Get()};
         bool success = true;