Move TaxTypeInterface::getDisplayLabel() to the tax zone level

Right now tax types have a display label. This label is used for the adjustment and is the same regardless of the selected zone and rate. However, #2874051: Create a tax type plugin for Canadian sales tax has a need for different display labels for different zones (or rates).

Discussed this with mglaman, we initially wanted to move the display label to TaxRate::getDisplayLabel(), but feared it would get repetitive (especially in the EU plugin) since most tax rates do share the same display label.
Decided to move it to TaxZone::getDisplayLabel() instead, which solves the Canadian problem, increasing the granularity, while not being too granular. If needed in the future we can expand to tax rates as well, and have an override system, but for now this is enough.