Add ParseRange function to parse args of the form int/[int,int]

src/rpc/util.h

@@ -38,6 +38,9 @@ unsigned int ParseConfirmTarget(const UniValue& value);
 RPCErrorCode RPCErrorFromTransactionError(TransactionError terr);
 UniValue JSONRPCTransactionError(TransactionError terr, const std::string& err_string = "");
 
+//! Parse a JSON range specified as int64, or [int64, int64]
+std::pair<int64_t, int64_t> ParseRange(const UniValue& value);
+
 struct RPCArg {
     enum class Type {
         OBJ,