New macro for named JSON convertor generation

[radistmorse]

This patch adds 4 new NLOHMANN_DEFINE_TYPE_<***>_WITH_NAMES macros. They behave the same way as the ones that are there, but require explicit JSON names. Useful for the situation when the fields in the class are following the naming convention that you do not want to expose to JSON, e.g.

    class address {
      private:
        std::string m_street;
        int m_housenumber;
        int m_postcode;

      public:
        NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES(address, "street", m_street, "housenumber", m_housenumber, "postcode", m_postcode)
    };

or if the name in JSON cannot be used as the field name because it is reserved, e.g.

    struct weird {
        std::string static_comp;
        std::string class_type;
        std::string public_field;
    };
    NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES(weird, "static", static_comp, "class", class_type, "public", public_field)

Also, this patch includes the unit tests for the new macros, and the update for the README to describe the new and existing macros better.

---

Pull request checklist

Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

[coveralls]

coverage: 100.0%. remained the same when pulling 0c4d816 on radistmorse:patch-1 into 5d27543 on nlohmann:develop.

[nlohmann]

Please update to the latest develop branch and resolve conflicts.

[github-actions]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @radistmorse
Please read and follow the Contribution Guidelines.

[Hackerl]

This feature is very good, when can it be merged?

[nlohmann]

When the CI is not failing any more.