Extract `account_table_ds` private method

[janko]

As discussed in https://github.com/jeremyevans/rodauth/discussions/267, this is useful when sharing database tables between different Rodauth configurations, and wanting to store the account type alongside account records, as it allows filtering retrieved accounts in a single place.

Example usage:

plugin :rodauth, name: :admin do
  enable :create_account, :login

  before_create_account do
    account[:type] = "admin"
  end

  auth_class_eval do
    def account_table_ds
      super.where(type: "admin")
    end
  end
end

[jeremyevans]

Thanks for the patch. This looks good. I'll try to test and merge today, but it might slip to next week.