Add RestlikeMessage codable model

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"
+  }
+}

Tests/ElixxirDAppsSDKTests/RestlikeMessageTests.swift

+import CustomDump
+import XCTest
+@testable import ElixxirDAppsSDK
+
+final class RestlikeMessageTests: XCTestCase {
+  func testCoding() throws {
+    let headers = secureRandomData(count: 32)
+    let content = secureRandomData(count: 32)
+    let jsonString = """
+    {
+      "Version": 1,
+      "Headers": \(headers.jsonEncodedBase64()),
+      "Content": \(content.jsonEncodedBase64()),
+      "Method": 2,
+      "URI": "xx://CmixRestlike/rest",
+      "Error": ""
+    }
+    """
+    let jsonData = jsonString.data(using: .utf8)!
+    let decoder = JSONDecoder()
+    decoder.dataDecodingStrategy = .base64
+    let message = try decoder.decode(RestlikeMessage.self, from: jsonData)
+
+    XCTAssertNoDifference(message, RestlikeMessage(
+      version: 1,
+      headers: headers,
+      content: content,
+      method: 2,
+      uri: "xx://CmixRestlike/rest",
+      error: ""
+    ))
+
+    let encoder = JSONEncoder()
+    encoder.dataEncodingStrategy = .base64
+    let encodedMessage = try encoder.encode(message)
+    let decodedMessage = try decoder.decode(RestlikeMessage.self, from: encodedMessage)
+
+    XCTAssertNoDifference(decodedMessage, message)
+  }
+}