psbt: add PSBTv2 global tx fields

When decomposing a transaction into a PSBTv2, the tx version and
locktime need to be stored in their respective global fields. Add those
fields and fill them when constructing.

src/psbt.cpp

@@ -16,6 +16,8 @@ using common::PSBTError;
 
 PartiallySignedTransaction::PartiallySignedTransaction(const CMutableTransaction& tx) : tx(tx)
 {
+    tx_version = tx.version;
+    fallback_locktime = tx.nLockTime;
     inputs.reserve(tx.vin.size());
     for (const CTxIn& input : tx.vin) {
         inputs.emplace_back(GetVersion(), input.prevout.hash, input.prevout.n, input.nSequence);

src/psbt.h

@@ -1073,6 +1073,9 @@ public:
     std::map<std::vector<unsigned char>, std::vector<unsigned char>> unknown;
     std::set<PSBTProprietary> m_proprietary;
 
+    uint32_t tx_version;
+    std::optional<uint32_t> fallback_locktime;
+
     bool IsNull() const;
     uint32_t GetVersion() const;
 
@@ -1203,6 +1206,8 @@ public:
                             throw std::ios_base::failure("Unsigned tx does not have empty scriptSigs and scriptWitnesses.");
                         }
                     }
+                    tx_version = tx->version;
+                    fallback_locktime = tx->nLockTime;
                     break;
                 }
                 case PSBT_GLOBAL_XPUB: