Remove problematic bigint encoding

This MR removes DHKey model and its problematic bigint encoding. This is a workaround until a better solution is developed. For now, the GroupMember model will be missing dhKey property.

Sources/XXClient/Models/GroupMember.swift

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