Add MessagePayload model

Examples/xx-messenger/Sources/AppCore/Models/MessagePayload.swift

+import Foundation
+
+public struct MessagePayload: Equatable {
+  public init(text: String) {
+    self.text = text
+  }
+
+  public var text: String
+}
+
+extension MessagePayload: Codable {
+  enum CodingKeys: String, CodingKey {
+    case text
+  }
+
+  public static func decode(_ data: Data) throws -> Self {
+    try JSONDecoder().decode(Self.self, from: data)
+  }
+
+  public func encode() throws -> Data {
+    try JSONEncoder().encode(self)
+  }
+}