Messenger receive file

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

Sources/XXMessengerClient/Messenger/Functions/MessengerRegisterReceiveFileCallback.swift

+import XCTestDynamicOverlay
+import XXClient
+
+public struct MessengerRegisterReceiveFileCallback {
+  public var run: (ReceiveFileCallback) -> Cancellable
+
+  public func callAsFunction(_ callback: ReceiveFileCallback) -> Cancellable {
+    run(callback)
+  }
+}
+
+extension MessengerRegisterReceiveFileCallback {
+  public static func live(_ env: MessengerEnvironment) -> MessengerRegisterReceiveFileCallback {
+    MessengerRegisterReceiveFileCallback { callback in
+      env.receiveFileCallbacksRegistry.register(callback)
+    }
+  }
+}
+
+extension MessengerRegisterReceiveFileCallback {
+  public static let unimplemented = MessengerRegisterReceiveFileCallback(
+    run: XCTUnimplemented("\(Self.self)", placeholder: Cancellable {})
+  )
+}