Include tier rewards for achievements

[andreasg123]

Now that some achievements have tier rewards, could you please add that to the API? Maybe it could be added to the tiers array as a reward property with the same data structure as the elements of the rewards array (or a rewards property with an array in case that a tier could provide multiple rewards). Here is an example for an achievement with tier rewards: https://api.guildwars2.com/v2/achievements/5035