Messenger receive file

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

Sources/XXMessengerClient/Messenger/Functions/MessengerStartFileTransfer.swift

+import XCTestDynamicOverlay
+import XXClient
+
+public struct MessengerStartFileTransfer {
+  public enum Error: Swift.Error, Equatable {
+    case notConnected
+  }
+
+  public var run: () throws -> Void
+
+  public func callAsFunction() throws -> Void {
+    try run()
+  }
+}
+
+extension MessengerStartFileTransfer {
+  public static func live(_ env: MessengerEnvironment) -> MessengerStartFileTransfer {
+    MessengerStartFileTransfer {
+      guard let e2e = env.e2e() else {
+        throw Error.notConnected
+      }
+      let fileTransfer = try env.initFileTransfer(
+        params: InitFileTransfer.Params(
+          e2eId: e2e.getId(),
+          e2eFileTransferParamsJSON: env.getE2EFileTransferParams(),
+          fileTransferParamsJSON: env.getFileTransferParams()
+        ),
+        callback: ReceiveFileCallback { result in
+          // TODO:
+        }
+      )
+      env.fileTransfer.set(fileTransfer)
+    }
+  }
+}
+
+extension MessengerStartFileTransfer {
+  public static let unimplemented = MessengerStartFileTransfer(
+    run: XCTUnimplemented("\(Self.self)")
+  )
+}