Add tiers wave reward section.

This commit introduces a new optional wave rewards section, `tiers`,
which allows server owners to configure rewards that "upgrade" over
time. This makes it possible to create reward configurations where
(some of) the rewards don't stack as the waves progress, but instead
change into different rewards. The most obvious use case is wanting to
grant an armor set for beating a specific wave, but a _better_ set for
beating a later wave, without granting both sets.

Turns out this was a lot easier than I thought it would be. We didn't
even need a rework of anything, and the Things API didn't even need to
adapt in any way. As an added bonus, we managed to get the associated
GitHub issue wrapped up before it turned 8 years old. Neat!

Closes #346

changelog.md

@@ -13,6 +13,7 @@ These changes will (most likely) be included in the next version.
 ## [Unreleased]
 ### Added
 - MobArena now properly supports Vault economy providers registered after MobArena has started. This should make it possible to use custom economy providers that aren't built into Vault, such as those created with Denizen.
+- New wave rewards section `tiers` allows for _non-stacking_ reward tiers for beating certain waves. This allows for configuring reward sets that get "upgraded" as the waves progress, e.g. by granting a full leather armor set for beating wave 15, but beating wave 20 _replaces_ that leather armor set with an iron armor set.
 
 ### Changed
 - Recurrent waves can now be randomized. If two or more recurrent waves clash on wave number, frequency, _and_ priority, MobArena will now randomly pick between them. This should make it easier to create more varied and interesting wave setups without having to resort to only massively randomized default waves.