Add CanFinalizeWithDefault method for partial records

[soareschen]

Example as asked on Reddit:

use cgp::core::field::CanFinalizeWithDefault;
use cgp::prelude::*;

#[derive(Default)]
pub struct Uuid;

#[derive(Default)]
pub struct ItemType;

#[derive(Default)]
pub struct EquipmentType;

#[derive(Default)]
pub struct ItemQuantity(pub u64);

#[derive(BuildField)]
pub struct Item {
    pub id: Uuid,
    pub name: String,
    pub item_type: ItemType,
    pub equipment_type: EquipmentType,
    pub maximum_quantity: ItemQuantity,
}

#[test]
fn test_default_construction() {
    let item = Item::builder()
        .build_field(PhantomData::<symbol!("name")>, "my-item".to_owned())
        .build_field(
            PhantomData::<symbol!("maximum_quantity")>,
            ItemQuantity(100),
        )
        .finalize_with_default();
}