Messenger - send & receive messages

• Add MessengerRegisterMessageListener function
• Add MessengerListenForMessages function
• Add MessengerSendMessage function

Sources/XXMessengerClient/Messenger/Functions/MessengerListenForMessages.swift

+import XCTestDynamicOverlay
+import XXClient
+
+public struct MessengerListenForMessages {
+  public enum Error: Swift.Error {
+    case notConnected
+  }
+
+  public var run: () throws -> Void
+
+  public func callAsFunction() throws -> Void {
+    try run()
+  }
+}
+
+extension MessengerListenForMessages {
+  public static func live(_ env: MessengerEnvironment) -> MessengerListenForMessages {
+    MessengerListenForMessages {
+      guard let e2e = env.e2e() else {
+        throw Error.notConnected
+      }
+      try e2e.registerListener(
+        senderId: nil,
+        messageType: 2,
+        callback: env.messageListeners.registered()
+      )
+    }
+  }
+}
+
+extension MessengerListenForMessages {
+  public static let unimplemented = MessengerListenForMessages(
+    run: XCTUnimplemented("\(Self.self)")
+  )
+}