Add MessengerRegisterForNotifications function

Sources/XXMessengerClient/Messenger/Functions/MessengerRegisterForNotifications.swift

+import Foundation
+import XXClient
+import XCTestDynamicOverlay
+
+public struct MessengerRegisterForNotifications {
+  public enum Error: Swift.Error, Equatable {
+    case notConnected
+  }
+
+  public var run: (Data) throws -> Void
+
+  public func callAsFunction(token: Data) throws -> Void {
+    try run(token)
+  }
+}
+
+extension MessengerRegisterForNotifications {
+  public static func live(_ env: MessengerEnvironment) -> MessengerRegisterForNotifications {
+    MessengerRegisterForNotifications { token in
+      guard let e2e = env.e2e() else {
+        throw Error.notConnected
+      }
+      try env.registerForNotifications(
+        e2eId: e2e.getId(),
+        token: token.map { String(format: "%02hhx", $0) }.joined()
+      )
+    }
+  }
+}
+
+extension MessengerRegisterForNotifications {
+  public static let unimplemented = MessengerRegisterForNotifications(
+    run: XCTUnimplemented("\(Self.self)")
+  )
+}