type-function.h: Fix CustomBuildField overload

Fix std::function CustomBuildField overload which is incompatible with a recent
change in 3a96cdc from
bitcoin-core#172 which changed
generated IPC client code to pass it an rvalue std::function reference instead
of an lvalue reference.

There was no test coverage for the type-function.h header earlier but the next
commit adds a test which would have caught the problem in the CustomBuildField
declaration.

Motivation for this change is to avoid a build error in
bitcoin/bitcoin#29409 when rebased on top of
bitcoin/bitcoin#32641 which includes
bitcoin-core#172

Author: ryanofsky

include/mp/type-function.h

@@ -24,7 +24,7 @@ template <typename Value, typename FnR, typename... FnParams, typename Output>
 void CustomBuildField(TypeList<std::function<FnR(FnParams...)>>,
     Priority<1>,
     InvokeContext& invoke_context,
-    Value& value,
+    Value&& value,
     Output&& output)
 {
     if (value) {