Workaround for big numbers in JSONs

This MR introduces a workaround for an issue where JSON models containing big numbers (eg 930-digits) can't be parsed by swift's JSONDecoder/JSONEncoder.

This affects the GroupMember model, which holds DHKey models that has value property with such a big number. The model's value is stored as String, and it's automatically decoded/encoded to big number in JSON.

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")
+  }
+}