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/GroupMember.swift

 import Foundation
 
 public struct GroupMember: Equatable {
-  public init(id: Data) {
+  public init(id: Data, dhKey: DHKey) {
     self.id = id
+    self.dhKey = dhKey
   }
 
   public var id: Data
+  public var dhKey: DHKey
 }
 
-extension GroupMember: Decodable {
+extension GroupMember: Codable {
   enum CodingKeys: String, CodingKey {
     case id = "ID"
+    case dhKey = "DhKey"
   }
 
   public static func decode(_ data: Data) throws -> Self {
-    let data = convertJsonNumberToString(in: data, minNumberLength: 19)
+    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")
+  }
 }
 
 extension Array where Element == GroupMember {
   public static func decode(_ data: Data) throws -> Self {
-    let data = convertJsonNumberToString(in: data, minNumberLength: 19)
+    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")
+  }
 }