Add DHKey model

Sources/XXClient/Models/DHKey.swift

+import Foundation
+
+public struct DHKey: Equatable {
+  public init(value: String, fingerprint: UInt64) {
+    self.value = value
+    self.fingerprint = fingerprint
+  }
+
+  public var value: String
+  public var fingerprint: UInt64
+}
+
+extension DHKey: Codable {
+  enum CodingKeys: String, CodingKey {
+    case value = "Value"
+    case fingerprint = "Fingerprint"
+  }
+
+  public static func decode(_ data: Data) throws -> Self {
+    let data = convertJsonNumberToString(in: data, at: "Value")
+    return try JSONDecoder().decode(Self.self, from: data)
+  }
+
+  public func encode() throws -> Data {
+    let data = try JSONEncoder().encode(self)
+    return convertJsonStringToNumber(in: data, at: "Value")
+  }
+}

Tests/XXClientTests/Models/DHKeyTests.swift

+import CustomDump
+import XCTest
+@testable import XXClient
+
+final class DHKeyTests: XCTestCase {
+  func testCoding() throws {
+    let value = "1759426033802606996617132861414734059978289057332806031357800676138355264622676606691435603603751978195460163638145821347601916259127578968570412642641025630452893097179266022832268525346700655861033031712000288680395716922501450233258587788020541937373196899001184700899008948530359980753630443486308876999029238453979779103124291315202352475056237021681172884599194016245219278368648568458514708567045834427853469072638665888791358582182353417065794210125797368093469194927663862565508608719835557592421245749381164023134450699040591219966988201315627676532245052123725278573237006510683695959381015415110970848376498637637944431576313526294020390694483472829278364602405292767170719547347485307956614210210673321959886410245334772057212077704024337636501108566655549055129066343309591274727538343075929837698653965640646190405582788894021694347212874155979958144038307500444865955516612526623220973497735316081265793063949"
+    let fingerprint: UInt64 = 15989433043166758754
+    let jsonString = """
+    {
+      "Value": \(value),
+      "Fingerprint": \(fingerprint)
+    }
+    """
+    let jsonData = jsonString.data(using: .utf8)!
+    let model = try DHKey.decode(jsonData)
+
+    XCTAssertNoDifference(model, DHKey(
+      value: value,
+      fingerprint: fingerprint
+    ))
+
+    let encodedModel = try model.encode()
+    let encodedModelString = String(data: encodedModel, encoding: .utf8)
+
+    XCTAssertNoDifference(
+      encodedModelString,
+      #"{"Value":\#(value),"Fingerprint":\#(fingerprint)}"#
+    )
+  }
+}