Supply value_type typedef in json_arrayt for STL algorithm usage.

Some of the STL algorithms require the `value_type` typedef on containers. Adding this typedef allows these algorithms to be used with `json_arrayt`. The following example will not compile without this commit.
```
  std::vector<std::string> example_strings = {"green", "eggs", "ham"};
  json_arrayt json_array;
  std::transform(
    example_strings.cbegin(),
    example_strings.cend(),
    std::back_inserter(json_array),
    [](std::string string) { return json_stringt(string); });
```

Author: thomasspriggs

src/util/json.h

@@ -169,6 +169,8 @@ class json_arrayt:public jsont
     array.push_back(jsont());
     return array.back();
   }
+
+  typedef jsont value_type; // NOLINT(readability/identifiers)
 };
 
 class json_stringt:public jsont