Allow adjustment types to be declared as "taxable"

Landing from a related issue, I hit a problem with tax and a new adjustment type.

I'm unsure if #5 and depending on weight vs. adding taxable is a more considerable change.

But as I see it, both are BC, because we would need to change how the price is collected prior to applying taxes
$adjusted_total_price = $order_item->getAdjustedTotalPrice(['promotion', 'fee']);

#5 with weight could be a more straightforward approach. Still, I am unsure if contrib modules are considered tax when declaring the weight of adjustment (I did not think of taxes when creating the adjustment type with weight 10).

Adding taxable property seems as more self-documented approach.

When getting the adjusted total price, even if we'd consider other adjustment types, it'd still be dependent on the priority of order processors... In other words, if an adjustment hasn't been added yet by a processor, the adjustments it may add won't be considered by the tax logic as these wouldn't be present.