feat: Allow decoding for types without default constructor

[SGSSGene]

This PR is inspired by #1010, but instead of creating a new customization point via decode_dispatcher it reuses convert by using convert<std::optional<T>> specialization for non-default constructible classes.

If cmake variable YAML_CPP_USE_OPTIONAL is set (automatically set if target is c++17 or above) this new feature is activated. It can be deactivated by setting YAML_CPP_USE_OPTIONAL to false to force old behavior if desired.

When decoding a type T, it will use convert<std::optional<T>> and forward to convert<T> if not available.
The signature requirements are as before but by using the additional std::optional it allows us to delay the construction of our type T.
Assume we have some type without default constructor:

class Vec3 {
  double x, y, z;
public:
  Vec3(double x, double y, double z} : x{x}, y{y}, z{z} {}
};

you could write

namespace YAML {
template<>
struct convert<std::optional<Vec3>> {
  static bool decode(const Node& node, std::optional<Vec3>& rhs) {
    if(!node.IsSequence() || node.size() != 3) {
      return false;
    }
    rhs.emplace(
        node[0].as<double>(),
        node[1].as<double>(),
        node[2].as<double>()
    );
    return true;
  }
};
}

To implement encoding for Vec3 one must still implement convert<Vec3> with the encode method.

Notes:
fixes #973 #993
alternative for PR #1010 and #1087

PR #1087 breaks API and relies on exception paths which I don't like, since this seems like normal control flow to me.

[SGSSGene]

Maybe it also enough to consider it as a fix for #506