ItemAPI tier fix

[Nebby1999]

Update 1.2.3 to Risk of Rain made the itemDef's "tier" into a property (Was originally a field). the Property's Set method causes issues when the itemTier catalog as not initialized yet (set method calls ItemTierCatalog.GetItemTierDef(itemTierEnum), and at that point no itemTierDefs exist in the catalog yet.)

This update fixes this issue by doing the following check on the tierargument, while also adding a new optional argument (For itemTierDefs)

When the tier argument is not assignedAtRuntime, the ItemDef's _itemTierDef gets loaded via Addressables by doing a switch case on the given tier argument.

If the tier argument is assignedAtRuntime, but no itemTierDef is specified, R2API throws a warning that the itemDef has the tier argument to assignedAtRuntime, but itemTierDef argument is null, afterwards it sets the ItemDef._itemTierDef to null (AKA: No tier)

if both the tier is AssignedAtRuntime, and the itemTierDef argument is not null, then it simpplpy sets the itemDef's _itemTierDef to the argument's value

I am unsure if this counts as a breaking change, some testing is needed

[Nebby1999]

I just tried to fix this by adding 2 overloads to the constructor and having the actual construction run in a private method, may not be the cleanest way of fixing the breakage but i cant figure out another one atm