[AltStoreCore] Adds NSManagedObjectContext.performAsync() to wrap iOS 15+ async perform()

Allows us to use Swift Concurrency with Core Data pre-iOS 15.

AltStoreCore/Extensions/AltStore+Async.swift

@@ -0,0 +1,61 @@
+//
+//  AltStore+Async.swift
+//  AltStoreCore
+//
+//  Created by Riley Testut on 3/23/23.
+//  Copyright © 2023 Riley Testut. All rights reserved.
+//
+
+import UIKit
+import CoreData
+
+public extension NSManagedObjectContext
+{
+    // Non-Throwing
+    func performAsync<T>(_ closure: @escaping () -> T) async -> T
+    {
+        let result: T
+        
+        if #available(iOS 15, *)
+        {
+            result = await self.perform(schedule: .immediate) {
+                closure()
+            }
+        }
+        else
+        {
+            result = await withCheckedContinuation { (continuation: CheckedContinuation<T, Never>) in
+                self.perform {
+                    let result = closure()
+                    continuation.resume(returning: result)
+                }
+            }
+        }
+        
+        return result
+    }
+    
+    // Throwing
+    func performAsync<T>(_ closure: @escaping () throws -> T) async throws -> T
+    {
+        let result: T
+        
+        if #available(iOS 15, *)
+        {
+            result = try await self.perform(schedule: .immediate) {
+                try closure()
+            }
+        }
+        else
+        {
+            result = try await withCheckedThrowingContinuation { (continuation: CheckedContinuation<T, Error>) in
+                self.perform {
+                    let result = Result { try closure() }
+                    continuation.resume(with: result)
+                }
+            }
+        }
+        
+        return result
+    }
+}