fix: add missing value_type to JSON iterators

ValueConstIterator and ValueIterator lacked the value_type typedef
required for std::iterator_traits deduction in C++20. GCC 13's STL
correctly diagnoses this when the iterators are used with algorithms
like std::all_of.

Fixes build on GCC 13 without requiring GCC 15.

include/xrpl/json/json_value.h

@@ -623,6 +623,7 @@ class ValueConstIterator : public ValueIteratorBase
 public:
     using size_t = unsigned int;
     using difference_type = int;
+    using value_type = Value const;
     using reference = Value const&;
     using pointer = Value const*;
     using SelfType = ValueConstIterator;
@@ -687,6 +688,7 @@ class ValueIterator : public ValueIteratorBase
 public:
     using size_t = unsigned int;
     using difference_type = int;
+    using value_type = Value;
     using reference = Value&;
     using pointer = Value*;
     using SelfType = ValueIterator;