Add UpdateBackupFunc callback

Sources/ElixxirDAppsSDK/Callbacks/UpdateBackupFunc.swift

+import Bindings
+import XCTestDynamicOverlay
+
+public struct UpdateBackupFunc {
+  public init(handle: @escaping (Data) -> Void) {
+    self.handle = handle
+  }
+
+  public var handle: (Data) -> Void
+}
+
+extension UpdateBackupFunc {
+  public static let unimplemented = UpdateBackupFunc(
+    handle: XCTUnimplemented("\(Self.self)")
+  )
+}
+
+extension UpdateBackupFunc {
+  func makeBindingsUpdateBackupFunc() -> BindingsUpdateBackupFuncProtocol {
+    class CallbackObject: NSObject, BindingsUpdateBackupFuncProtocol {
+      init(_ callback: UpdateBackupFunc) {
+        self.callback = callback
+      }
+
+      let callback: UpdateBackupFunc
+
+      func updateBackup(_ encryptedBackup: Data?) {
+        guard let encryptedBackup = encryptedBackup else {
+          fatalError("BindingsUpdateBackupFunc received `nil` encryptedBackup")
+        }
+        callback.handle(encryptedBackup)
+      }
+    }
+
+    return CallbackObject(self)
+  }
+}