Custom item spawning

[p1xel8ted]

Still to do:

• Remove reliance on asset bundle prefab for the sprite. Using TextureHelper, there are no errors, but there is also no (visible) sprite.
• Look at scaling when the item is on the offering shrine
• Remove/add check for debug for logs Add bool property to allow the author to specify if they want the item to be spawnable from offering shrines

[p1xel8ted]

• Simplified creating objectives using what's already available in the game.
• There are no localization methods to patch for objectives; they've done it all in a switch statement calling LocalizationManager directly. Postfixed the result had the same effect.
• ObjectiveKey must be visible to ensure that created objectives are tracked against the correct plugins when they make a new objective (required parameter in the constructor).
• Works fine in limited testing. Still need to test adding multiple different objectives.

[xhayper]

Can you mark this PR as a draft?