client: Add agent auto argument support

client: Add agent auto argument support

This adds "auto" capability which behaves like "on" but instead of
asking user to confirm/autorize it automatically accepts, which is
not secure to be used thus a warning is printed when user selects it.

Usage:

  [bluetoothctl]# agent auto
  Warning: setting auto response is not secure, it bypass user
  confirmation/authorization, it shall only be used for test automation.

  or

  client/bluetoothctl -a auto