[FEATURE REQUEST] Support std::optional, std::variant, and std::reference_wrapper

[jharmer95]

Feature Request: Support std::optional, std::variant, and std::reference_wrapper

Brief Summary of Request

std::optional, std::variant, and std::reference_wrapper are common standard library types that should be somewhat trivial to serialize

What Is The Value Added With This Feature

• Easier use for users whose API involves these standard library types or a type that uses these types
• Move closer to supporting most STL types

Possible Implementation

• JSON
  • optional
  • null or underlying value
  • variant
  • Use std::vist? (null for std::monostate)
  • reference_wrapper
  • deref
• bitsery
  • support already exists in bitsery 👍, just add if constexpr branches

Desired Outcome

// server
std::optional<std::string> Lookup(size_t key);

// client
auto result = client.template call_func<std::optional<std::string>>("Lookup", 241);

if (result.has_value())
{
    // ...
}

[jharmer95]

Added support for std::optional. std::variant, and std::reference_wrapper require some more tricky work

[jharmer95]

• std::variant is now supported (up to a fixed number of params, currently 10).
• std::reference_wrapper is also supported as it is the new interface to call "bound" RPCs explicitly
  • NOTE: other uses of std::reference_wrapper (i.e. std::vector<std::reference_wrapper>) are not supported