wallet,rpc: Add ParseOutputs function

Move the options parsing logic for SFFO out of FundTransaction into this
function.

Copy the options parsing logic from AddOutputs (called in
ConstructTransaction) into this function.

This sets us up to use ParseRecipients in the next commit and remove
SFFO logic from FundTransaction.

src/wallet/rpc/spend.cpp

@@ -77,6 +77,38 @@ std::set<int> ParseSubtractFeeFromOutputs(const UniValue& subtract_fee_from_outp
     return set_sffo;
 }
 
+std::vector<CRecipient> ParseOutputs(const UniValue& outputs_in, const UniValue& options)
+{
+    if (outputs_in.isNull()) {
+        throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, output argument must be non-null");
+    }
+
+    const bool outputs_is_obj = outputs_in.isObject();
+    UniValue outputs = outputs_is_obj ? outputs_in.get_obj() : outputs_in.get_array();
+
+    if (!outputs_is_obj) {
+        // Translate array of key-value pairs into dict
+        UniValue outputs_dict = UniValue(UniValue::VOBJ);
+        for (size_t i = 0; i < outputs.size(); ++i) {
+            const UniValue& output = outputs[i];
+            if (!output.isObject()) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, key-value pair not an object as expected");
+            }
+            if (output.size() != 1) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, key-value pair must contain exactly one key");
+            }
+            outputs_dict.pushKVs(output);
+        }
+        outputs = std::move(outputs_dict);
+    }
+    std::set<int> set_sffo;
+    if (options.exists("subtractFeeFromOutputs") || options.exists("subtract_fee_from_outputs") ) {
+        UniValue sffo = (options.exists("subtract_fee_from_outputs") ? options["subtract_fee_from_outputs"] : options["subtractFeeFromOutputs"]).get_array();
+        set_sffo = ParseSubtractFeeFromOutputs(sffo, outputs.getKeys());
+    }
+    return ParseRecipients(outputs, set_sffo);
+}
+
 static void InterpretFeeEstimationInstructions(const UniValue& conf_target, const UniValue& estimate_mode, const UniValue& fee_rate, UniValue& options)
 {
     if (options.exists("conf_target") || options.exists("estimate_mode")) {