Create utility RPCs for PSBT

decodepsbt takes a PSBT and decodes it to JSON

combinepsbt takes multiple PSBTs for the same tx and combines them.

finalizepsbt takes a PSBT and finalizes the inputs. If all inputs
are final, it extracts the network serialized transaction and returns
that instead of a PSBT unless instructed otherwise.

createpsbt is like createrawtransaction but for PSBTs instead of
raw transactions.

convertpsbt takes a network serialized transaction and converts it
into a psbt. The resulting psbt will lose all signature data and
an explicit flag must be set to allow transactions with signature
data to be converted.

src/core_read.cpp

@@ -7,6 +7,7 @@
 #include <primitives/block.h>
 #include <primitives/transaction.h>
 #include <script/script.h>
+#include <script/sign.h>
 #include <serialize.h>
 #include <streams.h>
 #include <univalue.h>
@@ -160,6 +161,23 @@ bool DecodeHexBlk(CBlock& block, const std::string& strHexBlk)
     return true;
 }
 
+bool DecodePSBT(PartiallySignedTransaction& psbt, const std::string& base64_tx, std::string& error)
+{
+    std::vector<unsigned char> tx_data = DecodeBase64(base64_tx.c_str());
+    CDataStream ss_data(tx_data, SER_NETWORK, PROTOCOL_VERSION);
+    try {
+        ss_data >> psbt;
+        if (!ss_data.empty()) {
+            error = "extra data after PSBT";
+            return false;
+        }
+    } catch (const std::exception& e) {
+        error = e.what();
+        return false;
+    }
+    return true;
+}
+
 uint256 ParseHashStr(const std::string& strHex, const std::string& strName)
 {
     if (!IsHex(strHex)) // Note: IsHex("") is false