Support discounts specifying fields in the discount UI

[ryanmitchell]

Currently custom discounts cant specify fields the filament UI.

This PR attempts to fix that by allowing discounts to specify 3 'magic' methods:

lunarPanelSchema(): array The Filament schema for the fields to be displayed when the discount type is specified

lunarPanelOnFill(array $data): array To allow the discount to modify the data associated with the discount before its rendered (only called if the discount type is selected)

lunarPanelOnSave(array $data): array To allow the discount to modify the data associated with the discount before its saved (only called if the discount type is selected)

Here is an example of how it is used, in a Amount off (Retail price) discount that we use in one of our systems:

    public function lunarPanelSchema(): array
    {
        $currencies = Currency::get();

        $currencyInputs = [];

        foreach ($currencies as $currency) {
            $currencyInputs[] = Forms\Components\TextInput::make(
                'data.retail.fixed_values.'.$currency->code
            )
            ->label($currency->name)
            ->afterStateHydrated(function (Forms\Components\TextInput $component, $state) use ($currencies) {
                $currencyCode = last(explode('.', $component->getStatePath()));
                $currency = $currencies->first(
                    fn ($currency) => $currency->code == $currencyCode
                );

                if ($currency) {
                    $component->state($state / $currency->factor);
                }
            });
        }

        return [
            Forms\Components\Toggle::make('data.retail.fixed_value')->live(),
            Forms\Components\TextInput::make('data.retail.percentage')->visible(
                fn (Forms\Get $get) => ! $get('data.retail.fixed_value')
            )->numeric(),
            Forms\Components\Group::make(
                $currencyInputs
            )->visible(
                fn (Forms\Get $get) => (bool) $get('data.retail.fixed_value')
            )->columns(3),
        ];
    }

    public function lunarPanelOnFill(array $data): array
    {
        if (! Arr::get($data, 'data.retail', false)) {
            $dataValues = $data['data'] ?? [];

            $dataValues['retail'] = [];

            if (isset($dataValues['fixed_value'])) {
                $dataValues['retail']['fixed_value'] = $dataValues['fixed_value'];
                unset($dataValues['fixed_value']);
            }

            if (isset($dataValues['fixed_values'])) {
                $dataValues['retail']['fixed_values'] = $dataValues['fixed_values'];
                unset($dataValues['fixed_values']);
            }

            if (isset($dataValues['percentage'])) {
                $dataValues['retail']['percentage'] = $dataValues['percentage'];
                unset($dataValues['percentage']);
            }

            $data['data'] = $dataValues;
        }

        return $data;
    }

    public function lunarPanelOnSave(array $data): array
    {
        if ($retail = Arr::get($data, 'data.retail', false)) {
            $data['data'] = array_merge($data['data'], $retail);
            unset($data['data']['retail']);

            $currencies = Currency::enabled()->get();
            $fixedPrices = $data['data']['fixed_values'] ?? [];
            foreach ($fixedPrices as $currencyCode => $fixedPrice) {
                $currency = $currencies->first(
                    fn ($currency) => $currency->code == $currencyCode
                );

                if (! $currency) {
                    continue;
                }
                $data['data']['fixed_values'][$currencyCode] = (int) round($fixedPrice * $currency->factor);
            }
        }

        return $data;
    }

As you can see we are using some of the same data values as the standard "Amount off" discount, but we have to prefix them in the schema to avoid collisions on the filament side... then on fill/save we add/undo those prefixes so they dont get saved to the database. This is just a simple example to show how they can used and why they are necessary.

The benefit of this approach is that the discount is just registered once, making the DX experience simpler.

        Discounts::addType(
            DiscountTypes\AmountOffRetail::class
        );