Messenger receive file

• Add MessengerRegisterReceiveFileCallback function.
• Add MessengerStartFileTransfer function.
• Update MessengerSendFile function.
• Add MessengerReceiveFile function.

Sources/XXMessengerClient/Messenger/Functions/MessengerReceiveFile.swift

+import Foundation
+import XCTestDynamicOverlay
+import XXClient
+
+public struct MessengerReceiveFile {
+  public enum Error: Swift.Error, Equatable {
+    case notConnected
+  }
+
+  public var run: () throws -> Void
+
+  public func callAsFunction() throws -> Void {
+    try run()
+  }
+}
+
+extension MessengerReceiveFile {
+  public static func live(_ env: MessengerEnvironment) -> MessengerReceiveFile {
+    MessengerReceiveFile {
+      guard let e2e = env.e2e() else {
+        throw Error.notConnected
+      }
+      // TODO: implement receiving file
+    }
+  }
+}
+
+extension MessengerReceiveFile {
+  public static let unimplemented = MessengerReceiveFile(
+    run: XCTUnimplemented("\(Self.self)")
+  )
+}