Add missing overload to ordered_map::find

[nlohmann]

ordered_map provides four lookup variants for emplace, at, count, and the non-const find, but only three variants for const lookup: one exact-key non-const, one template non-const, and one exact-key const. The template const overload is absent:

// present
iterator find(const key_type& key);                    // exact, non-const
iterator find(KeyType&& key);                          // template, non-const  ✓
const_iterator find(const key_type& key) const;        // exact, const         ✓

// MISSING
const_iterator find(KeyType&& key) const;              // template, const      ✗

This PR adds the mission overload and a test.