Fixing product overrides

Fixes: #79

The problem was that when a customer option value had multiple overrides then, it would always take the last override no matter what product this is for.

This fix sadly breaks backwards compatibility. The method signature has changed:

    public function getPriceForChannel(
        ChannelInterface $channel,
+       ProductInterface $product,
        bool $ignoreActive = false
    ): ?CustomerOptionValuePriceInterface;