rpc: descriptorprocesspsbt return hex encoded tx

If processed psbt is complete return hex encoded network
transaction in the output.

src/rpc/rawtransaction.cpp

@@ -1949,6 +1949,7 @@ RPCHelpMan descriptorprocesspsbt()
                     {
                         {RPCResult::Type::STR, "psbt", "The base64-encoded partially signed transaction"},
                         {RPCResult::Type::BOOL, "complete", "If the transaction has a complete set of signatures"},
+                        {RPCResult::Type::STR_HEX, "hex", /*optional=*/true, "The hex-encoded network transaction if complete"},
                     }
                 },
                 RPCExamples{
@@ -1989,7 +1990,14 @@ RPCHelpMan descriptorprocesspsbt()
 
     result.pushKV("psbt", EncodeBase64(ssTx));
     result.pushKV("complete", complete);
-
+    if (complete) {
+        CMutableTransaction mtx;
+        PartiallySignedTransaction psbtx_copy = psbtx;
+        CHECK_NONFATAL(FinalizeAndExtractPSBT(psbtx_copy, mtx));
+        CDataStream ssTx_final(SER_NETWORK, PROTOCOL_VERSION);
+        ssTx_final << mtx;
+        result.pushKV("hex", HexStr(ssTx_final));
+    }
     return result;
 },
     };