Add GroupMessageHandler to messenger example

Examples/xx-messenger/Sources/AppCore/Groups/GroupMessageHandler.swift

+import XXModels
+import XXClient
+import Foundation
+import XXMessengerClient
+import XCTestDynamicOverlay
+
+public struct GroupMessageHandler {
+  public typealias OnError = (Error) -> Void
+
+  public var run: (@escaping OnError) -> Cancellable
+
+  public func callAsFunction(onError: @escaping OnError) -> Cancellable {
+    run(onError)
+  }
+}
+
+extension GroupMessageHandler {
+  public static func live(
+    messenger: Messenger,
+    db: DBManagerGetDB
+  ) -> GroupMessageHandler {
+    GroupMessageHandler { onError in
+      messenger.registerGroupChatProcessor(.init { result in
+        switch result {
+        case .success(let callback):
+          do {
+            let payload = try MessagePayload.decode(callback.decryptedMessage.payload)
+            try db().saveMessage(.init(
+              networkId: callback.decryptedMessage.messageId,
+              senderId: callback.decryptedMessage.senderId,
+              recipientId: nil,
+              groupId: callback.decryptedMessage.groupId,
+              date: Date(timeIntervalSince1970: TimeInterval(callback.decryptedMessage.timestamp) / 1_000_000_000),
+              status: .received,
+              isUnread: true,
+              text: payload.text,
+              replyMessageId: payload.replyingTo,
+              roundURL: callback.roundUrl
+            ))
+          } catch {
+            onError(error)
+          }
+        case .failure(let error):
+          onError(error)
+        }
+      })
+    }
+  }
+}
+
+extension GroupMessageHandler {
+  public static let unimplemented = GroupMessageHandler(
+    run: XCTestDynamicOverlay.unimplemented("\(Self.self)", placeholder: Cancellable {})
+  )
+}