Support serializing Enum values instead of names

[sarahlydia9868]

I propose adding a trait or macro similar to JSONCONS_ENUM_VALUE_TRAITS This feature would allow developers to explicitly define the serialized output value for each enum member.
Ex:

enum class PacketCompression : uint32_t {
        NONE = 199_u32,
        TEXTURE,
        GENERAL,
        ALL,
    };
    
JSONCONS_ENUM_VALUE_TRAITS(PacketCompression, NONE, TEXTURE, GENERAL, ALL);

Output: (PacketCompression = NONE)

{
   value: 199
}

[danielaparker]

This should be default behaviour now on master,

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

enum class PacketCompression  {
    NONE = 199,
    TEXTURE,
    GENERAL,
    ALL
};

struct Foo
{
    PacketCompression value;
};

JSONCONS_ALL_MEMBER_TRAITS(Foo,value)

int main()
{
    try
    {
        auto j = jsoncons::json::parse(R"({"value":199})");

        auto foo = j.as<Foo>();

        std::cout << "(1) " << (int)foo.value << "\n";

        std::string buffer;
        jsoncons::encode_json(foo, buffer);
        std::cout << "(2) " << buffer << "\n";
    }
    catch (const std::exception& e)
    {
        std::cout << e.what() << "\n";
    }
}

Output:

(1) 199
(2) {"value":199}