Bindings models wrapper

• Add Identity model.
• Add MessageSendReport model.
• Add Fact model.
• Add Message model.
• Add RestlikeMessage model.
• Add tests for models decoding and encoding (use example JSONs from Bindings as expected JSON schemas).

Sources/ElixxirDAppsSDK/Message.swift

+import Foundation
+
+public struct Message: Equatable {
+  public init(
+    messageType: Int,
+    id: Data,
+    payload: Data,
+    sender: Data,
+    recipientId: Data,
+    ephemeralId: Int,
+    timestamp: Int,
+    encrypted: Bool,
+    roundId: Int
+  ) {
+    self.messageType = messageType
+    self.id = id
+    self.payload = payload
+    self.sender = sender
+    self.recipientId = recipientId
+    self.ephemeralId = ephemeralId
+    self.timestamp = timestamp
+    self.encrypted = encrypted
+    self.roundId = roundId
+  }
+
+  public var messageType: Int
+  public var id: Data
+  public var payload: Data
+  public var sender: Data
+  public var recipientId: Data
+  public var ephemeralId: Int
+  public var timestamp: Int
+  public var encrypted: Bool
+  public var roundId: Int
+}
+
+extension Message: Codable {
+  enum CodingKeys: String, CodingKey {
+    case messageType = "MessageType"
+    case id = "ID"
+    case payload = "Payload"
+    case sender = "Sender"
+    case recipientId = "RecipientID"
+    case ephemeralId = "EphemeralID"
+    case timestamp = "Timestamp"
+    case encrypted = "Encrypted"
+    case roundId = "RoundId"
+  }
+}