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/RestlikeMessage.swift

+import Foundation
+
+public struct RestlikeMessage: Equatable {
+  public init(
+    version: Int,
+    headers: Data,
+    content: Data,
+    method: Int,
+    uri: String,
+    error: String
+  ) {
+    self.version = version
+    self.headers = headers
+    self.content = content
+    self.method = method
+    self.uri = uri
+    self.error = error
+  }
+
+  public var version: Int
+  public var headers: Data
+  public var content: Data
+  public var method: Int
+  public var uri: String
+  public var error: String
+}
+
+extension RestlikeMessage: Codable {
+  enum CodingKeys: String, CodingKey {
+    case version = "Version"
+    case headers = "Headers"
+    case content = "Content"
+    case method = "Method"
+    case uri = "URI"
+    case error = "Error"
+  }
+}