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/rpc/client.cpp

@@ -109,6 +109,14 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "combinerawtransaction", 0, "txs" },
     { "fundrawtransaction", 1, "options" },
     { "fundrawtransaction", 2, "iswitness" },
+    { "createpsbt", 0, "inputs" },
+    { "createpsbt", 1, "outputs" },
+    { "createpsbt", 2, "locktime" },
+    { "createpsbt", 3, "replaceable" },
+    { "combinepsbt", 0, "txs"},
+    { "finalizepsbt", 1, "extract"},
+    { "converttopsbt", 1, "permitsigdata"},
+    { "converttopsbt", 2, "iswitness"},
     { "gettxout", 1, "n" },
     { "gettxout", 2, "include_mempool" },
     { "gettxoutproof", 0, "txids" },