Support free function pointers in glz::custom

[stephenberry]

Resolves #2639

Free functions can now be used with glz::custom, following the same conventions as lambdas: the object is the first argument, the decoded input is the second, and an optional glz::context& third argument enables custom error handling. A free function with a non-void return acts as a getter, so reading decodes into the returned reference and writing serializes the returned value.

struct free_func_counter
{
   int count{};
};

void counter_read(free_func_counter& obj, int val) { obj.count = val; }
int counter_write(const free_func_counter& obj) { return obj.count; }

template <>
struct glz::meta<free_func_counter>
{
   static constexpr auto value = object("count", custom<counter_read, counter_write>);
};

Implementation notes

• invocable_args_t/invocable_result_t now dispatch through a new invocable_traits_for selector that handles function objects via decltype(&T::operator()) and free function pointers directly. Its primary template is intentionally empty so the aliases remain SFINAE-friendly for other types (e.g. std::function member pointers inspected inside std::enable_if_t conditions), matching the previous alias behavior.
• is_invocable_concrete is deliberately unchanged (it still means "callable with a concrete operator()"), so the dispatch chains in registry.hpp, cli_menu.hpp, and constraint.hpp are unaffected. The places that support free functions (custom.hpp, custom_meta.hpp, reflect.hpp) opt in explicitly with is_function_ptr_invocable.
• The custom read dispatch routes non-void concrete callables to the reference-read path instead of a blanket static_assert, so reference-returning free functions (and now concrete lambdas) work as read-side getters.
• custom_read_input_type recognizes free function setters, enabling variant type deduction and JSON schema generation for them, on par with lambdas.

Tests

New free_function_custom_tests suite in json_test covering: basic free read/write, the glz::context& read and write forms with custom error messages, mixing a free function with a member pointer, string inputs including escape round-tripping, and a reference-returning free function. Also verified compile-compatibility for registry member-function endpoints, registry function-pointer endpoints, whole-value custom over std::function members inside variants and schema generation, and cli_menu structs with multi-argument function pointer members.