Add interfaces for jwt-cpp compatibility (#479)

Co-authored-by: Ali Girayhan Ozbay <aligirayhan.ozbay@abex.capital>

Author: aligirayhanozbay

include/rfl/Object.hpp

@@ -90,6 +90,8 @@ class Object {
   /// The number of elements currently inside the object.
   auto size() const { return data_.size(); }
 
+  std::size_t count(const key_type& key) const { return std::count_if(cbegin(), cend(), [&](const auto& p) { return p.first == key; }); }
+
   /// The maximum possible size.
   auto max_size() const { return data_.max_size(); }