Add dynamic set and get.

[AndreaCatania]

This allow to have components (and shared components) with variable properties, which is useful to expose arrays, or structure with dynamic properites.

To use this featues it's necessary to implement the functions:

• bool _set(const StringName &p_name, const Variant &p_value);
• bool _get(const StringName &p_name, Variant &r_value) const;
• void _get_property_list(List<PropertyInfo> *r_list) const;

Here an example:

/// This Component accepts a color property on if the paramter `enable_color` is true.
struct TestDynamicSetGetComponent {
    COMPONENT(TestDynamicSetGetComponent, DenseVectorStorage);

    static void _bind_methods() {
        ECS_BIND_PROPERTY(TestDynamicSetGetComponent, PropertyInfo(Variant::BOOL, "enable_color"), enable_color);
    }

    void _get_property_list(List<PropertyInfo> *r_list) {
        if (enable_color) {
            r_list->push_back(PropertyInfo(Variant::COLOR, "color"));
        }
    }

    bool _set(const StringName &p_name, const Variant &p_value) {
        if (enable_color) {
            if (p_name == SNAME("color")) {
                meta[SNAME("color")] = p_value;
                return true;
            }
        }
        return false;
    }

    bool _get(const StringName &p_name, Variant &r_value) const {
        if (enable_color) {
            if (p_name == SNAME("color")) {
                if (meta.has(SNAME("color"))) {
                    r_value = meta[SNAME("color")];
                } else {
                    // Default color.
                    r_value = Color(1.0, 0.0, 0.0);
                }
                return true;
            }
        }
        return false;
    }

    bool enable_color = false;
    Dictionary meta;
};

https://user-images.githubusercontent.com/8342599/131245318-a4f055d2-93a8-4105-9d20-728c81996daf.mp4

---

Fixes: https://github.com/GodotECS/godex/issues/139

[AndreaCatania]

Implements: https://github.com/GodotECS/godex/discussions/236

[AndreaCatania]

Fixes: https://github.com/GodotECS/godex/issues/139