Internal purchase controller (only) refactor

[super-bryan]

• Allows an internal purchase controller (termed "automatic purchase controller") to be used identically to an external purchase controller
• Uncouples ReceiptManager logic from the original purchase controller
• Moves receipt logic from the StoreKitManager into the ReceiptManager
• Removes the need for if/else logic when it comes to purchase controller logic
• Uses the adapter pattern only when necessary (i.e. only for objc types)
• Makes classes more protocol-oriented to avoid tightly coupled classes