Add Nullable fields

[FxMorin]

This adds support for optional fields without needing to have your values wrapped in Optional.

Before:

private record Data(
    Optional<String> string,
    Optional<Integer> integer
) {
    public static final Codec<Data> CODEC = RecordCodecBuilder.create(i -> i.group(
            Codec.STRING.optionalFieldOf("string").forGetter(Data::string),
            Codec.INT.optionalFieldOf("integer").forGetter(Data::integer)
    ).apply(i, Data::new));
}

After:

private record Data(
    @Nullable String string,
    @Nullable Integer integer
) {
    public static final Codec<Data> CODEC = RecordCodecBuilder.create(i -> i.group(
            Codec.STRING.nullableFieldOf("string").forGetter(Data::string),
            Codec.INT.nullableFieldOf("integer").forGetter(Data::integer)
    ).apply(i, Data::new));
}

You can see the tests for more examples.

This was done since Optional is intended to be used as a method return type. Not as a field, nullability exists for that purpose.
For people following the standards and using annotations such as JSpecify with proper nullability tools, using Optional is just pointless wrapping that DFU is forcing you to use.