TaxedPrice: Add tax_amount property
TaxedPrice: Add tax_amount property

Refs SHOOP-975

jorge-marques/shoop,shoopio/shoop,jorge-marques/shoop,hrayr-artunyan/shuup,akx/shoop,hrayr-artunyan/shuup,taedori81/shoop,suutari/shoop,shoopio/shoop,suutari-ai/shoop,suutari-ai/shoop,jorge-marques/shoop,suutari/shoop,akx/shoop,shawnadelic/shuup,shoopio/shoop,taedori81/shoop,hrayr-artunyan/shuup,suutari-ai/shoop,shawnadelic/shuup,taedori81/shoop,suutari/shoop,akx/shoop,shawnadelic/shuup