Add ReceiveFileCallback

Sources/ElixxirDAppsSDK/Callbacks/ReceiveFileCallback.swift

+import Bindings
+import XCTestDynamicOverlay
+
+public struct ReceiveFileCallback {
+  public var handle: (Result<ReceivedFile, NSError>) -> Void
+}
+
+extension ReceiveFileCallback {
+  public static let unimplemented = ReceiveFileCallback(
+    handle: XCTUnimplemented("\(Self.self)")
+  )
+}
+
+extension ReceiveFileCallback {
+  func makeBindingsReceiveFileCallback() -> BindingsReceiveFileCallbackProtocol {
+    class CallbackObject: NSObject, BindingsReceiveFileCallbackProtocol {
+      init(_ callback: ReceiveFileCallback) {
+        self.callback = callback
+      }
+
+      let callback: ReceiveFileCallback
+
+      func callback(_ payload: Data?, err: Error?) {
+        if let error = err {
+          callback.handle(.failure(error as NSError))
+        } else if let data = payload {
+          do {
+            callback.handle(.success(try ReceivedFile.decode(data)))
+          } catch {
+            callback.handle(.failure(error as NSError))
+          }
+        } else {
+          fatalError("BindingsReceiveFileCallback received `nil` payload and `nil` error")
+        }
+      }
+    }
+
+    return CallbackObject(self)
+  }
+}