How do I get the hero and purifier bonuses when capturing shadow Pokemon?
The two bonuses correspond to two new badges of the same names added to the game:
Hero – Defeat 10 / 100 / 1000 Team GO Rocket Grunts
Purifier – Purify 5 / 50 / 500 Shadow Pokemon
Having these badges will grant you the bonus balls, with additional levels in the badge gaining additional extra balls.
(Source for the badge levels; Source for the bonuses.)