boost::json::serializer should take a storage_ptr

[pdimov]

When converting a JSON value to a string, it's sometimes necessary to use a specific storage_ptr for the temporary allocations done by the json::serializer. One such example in https://github.com/pdimov/mustache/blob/develop/src/renderer.cpp#L849. The Mustache library does all its temporary allocations with a user-provided storage_ptr, so it would be nice if I can give the serializer that pointer.