Simplify Tax Calculation for custom adjustments

[solverat]

Q	A
Bug report?	no
Feature request?	yes
BC Break report?	no
RFC?	yes

Improvement: Simplify Tax Calculation for custom adjustments.

Currently, if you're adding an additional (positive) adjustment to the cart item, you manually have to call the tax calculator:

$withTax = true;
$taxCalculator = $this->taxCalculatorFactory->getTaxCalculator(
    $item->getProduct(),
    $cart->getShippingAddress() ?: $this->defaultAddressProvider->getAddress($cart),
    $context,
);

if ($taxCalculator instanceof TaxCalculatorInterface) {

    $taxItems = $item->getTaxes() ?? new Fieldcollection();

    if ($withTax) {
        $taxItems->setItems(
            $this->taxCollector->collectTaxesFromGross(
                $taxCalculator,
                $amountGrossTotal,
                $taxItems->getItems(),
            ),
        );
    } else {
        $taxItems->setItems(
            $this->taxCollector->collectTaxes(
                $taxCalculator,
                $amountNetTotal,
                $taxItems->getItems(),
            ),
        );
    }
}

$item->addAdjustment(
    $this->adjustmentFactory->createWithData(
        'my_adjustment',
        '',
        $amountGrossTotal,
        $amountNetTotal,
    )
);

This is basically the same thing as CartItemRuleApplier and CartItemRuleApplier does.

Proposal: Add a service which handles those things for us.