[C++][Pistache] Add compatibility for nlohmann-json 3.5.0

Created by: muttleyxd

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Description of the PR

While generated sample downloads nlohmann-json from GitHub, it could also be provided by operating system. Debian 10 Buster provides nlohmann-json3-dev package with 3.5.0 version https://packages.debian.org/buster/nlohmann-json3-dev.

contains was added in 3.6.0 https://github.com/nlohmann/json/releases/tag/v3.6.0

This change makes generated code compatible with 3.5.0.

Here's the code I tested it with:

#include <nlohmann/json.hpp>
#include <iostream>

void check_with_find(char const * const key, nlohmann::json const &json)
{
  if (json.find(key) != json.end())
    std::cout << "find: " << key << " exists!\n";
  else
    std::cout << "find: " << key << " does not exist!\n";
}

void check_with_contains(char const * const key, nlohmann::json const &json)
{
  if (json.contains(key))
    std::cout << "contains: " << key << " exists!\n";
  else
    std::cout << "contains: " << key << " does not exist!\n";
}

int main()
{
  constexpr char const * existing_key = "hello";
  constexpr char const * not_existing_key = "there";

  nlohmann::json json;  
  json[existing_key] = "general";

  check_with_find(existing_key, json);
  check_with_find(not_existing_key, json);
  check_with_contains(existing_key, json);
  check_with_contains(not_existing_key, json);
}

Output:

$ g++ -std=c++17 -Wall -Wextra -pedantic json-find.cpp -o json-find
$ ./json-find
find: hello exists!
find: there does not exist!
contains: hello exists!
contains: there does not exist!

contains is implemented as:

template<typename KeyT>
bool contains(KeyT&& key) const
{
    return is_object() and m_value.object->find(std::forward<KeyT>(key)) != m_value.object->end();
}

So it's almost the same code