static begin ({ bookmarks, txConfig, database, mode, impersonatedUser, notificationFilter } = {}) {
    const metadata = buildTxMetadata(bookmarks, txConfig, database, mode, impersonatedUser, notificationFilter)
    return new RequestMessage(
      BEGIN,
      [metadata],
      () => `BEGIN ${json.stringify(metadata)}`
    )
  }
Create a new BEGIN message.
@param {Bookmarks} bookmarks the bookmarks.
@param {TxConfig} txConfig the configuration.
@param {string} database the database name.
@param {string} mode the access mode.
@param {string} impersonatedUser the impersonated user.
@param {NotificationFilter} notificationFilter the notification filter
@return {RequestMessage} new BEGIN message.