iap: move StoreKit logic out of DamusPurpleView and into a new PurpleStoreKitManager

This commit moves most of StoreKit-specific logic that was embedded into
DamusPurpleView and places it into a new PurpleStoreKitManager struct,
to make code more reusable and readable by separating view concerns from
StoreKit-specific concerns.

Most of the code here should be in feature parity with the previous
behavior. However, a few logical improvements were made alongside this
refactoring:

- Improved StoreKit transaction update monitoring logic: Previously the
  view would stop listening for purchase updates after the first update.
  However, I made the program continuously listen for purchase updates,
  as recommended by Apple's documentation
  (https://developer.apple.com/documentation/storekit/transaction/3851206-updates)

- Improved/simplified logic around getting extra information from the
  products: Information and the handling of product information was
  spread in a few separate places. I incorporated those bits of
  information into central and uniform interfaces on DamusPurpleType, to
  simplify logic and future changes.

Signed-off-by: Daniel D’Aquino <daniel@daquino.me>
Reviewed-by: William Casarin <jb55@jb55.com>
Signed-off-by: William Casarin <jb55@jb55.com>

damus/Models/Purple/DamusPurple.swift

@@ -6,10 +6,12 @@
 //
 
 import Foundation
+import StoreKit
 
 class DamusPurple: StoreObserverDelegate {
     let settings: UserSettingsStore
     let keypair: Keypair
+    var storekit_manager: StoreKitManager
 
     @MainActor
     var account_cache: [Pubkey: Account]
@@ -18,6 +20,7 @@ class DamusPurple: StoreObserverDelegate {
         self.settings = settings
         self.keypair = keypair
         self.account_cache = [:]
+        self.storekit_manager = .init()
     }
     
     // MARK: Functions
@@ -123,6 +126,10 @@ class DamusPurple: StoreObserverDelegate {
         try await self.create_account(pubkey: pubkey)
     }
     
+    func make_iap_purchase(product: Product) async throws -> Product.PurchaseResult {
+        return try await self.storekit_manager.purchase(product: product)
+    }
+    
     func send_receipt() async {
         // Get the receipt if it's available.
         if let appStoreReceiptURL = Bundle.main.appStoreReceiptURL,