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/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: Decodable {
-  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)
-  }
-}