message: feat(core): add `ecdhXprv` to serialized SDK JSON object
  Ticket:
diff: @@ -85,6 +85,7 @@ export interface BitGoJson {
  user?: User;
  token?: string;
  extensionKey?: string;
  + ecdhXprv?: string;
  }
  /**
  @@ -684,6 +685,7 @@ export class BitGo {
  user: this._user,
  token: this._token,
  extensionKey: this._extensionKey ? this._extensionKey.toWIF() : undefined,
  + ecdhXprv: this._ecdhXprv,
  };
  }
  @@ -698,6 +700,7 @@ export class BitGo {
  fromJSON(json: BitGoJson): void {
  this._user = json.user;
  this._token = json.token;
  + this._ecdhXprv = json.ecdhXprv;
  if (json.extensionKey) {
  const network = common.Environments[this.getEnv()].network;
  this._extensionKey = utxolib.ECPair.fromWIF(
  