ItemAPI ItemDisplay fixes

[Nebby1999]

This PR fixes issue #370.

ItemAPI no longer checks if the pickup model prefab contains the ItemDisplay component, as in reality that prefab does not require said component, but instead checks the followerPrefab for the rules in the item/equipment's ItemDisplayDictionary.

The ItemDisplayDictionary now has a Validation method, the validation method does not prohibit the addition of a new item/equipment, instead, it's sole purpose is to ensure the Dictionary has valid ItemDisplayRules, the validation method logs the faulty rules using a string builder, which is later logged into the console using R2API.Logger.DebugLog().

The validator looks for:
ItemDisplayRules where the follower prefab is null
ItemDisplayRules where the follower prefab is not null, but does not have an ItemDisplay component
ItemDisplayRules where the follower prefab is not null, has an ItemDisplay Component, but said component's RendererInfos is empty.

[tristanmcpherson]

Looks good