Add RegisterForNotifications functor

Sources/ElixxirDAppsSDK/Functors/RegisterForNotifications.swift

+import Bindings
+import XCTestDynamicOverlay
+
+public struct RegisterForNotifications {
+  public var run: (Int, String) throws -> Void
+
+  public func callAsFunction(
+    e2eId: Int,
+    token: String
+  ) throws {
+    try run(e2eId, token)
+  }
+}
+
+extension RegisterForNotifications {
+  public static let live = RegisterForNotifications { e2eId, token in
+    var error: NSError?
+    BindingsRegisterForNotifications(e2eId, token, &error)
+    if let error = error {
+      throw error
+    }
+  }
+}
+
+extension RegisterForNotifications {
+  public static let unimplemented = RegisterForNotifications(
+    run: XCTUnimplemented("\(Self.self)")
+  )
+}