The patch adds a new supported option "offline" to the do_transaction
method of the Goal interface. If set to "true", the transaction is not
executed, but only prepared to run during the next reboot.

Adds new `Offline` interface for interacting with offline transactions.
The interface provides three methods:

- check_pending() - check whether there is an offline update scheduled
  for the next reboot

- cancel() - cancel the scheduled offline update

- clean() - cancel the scheduled offline update and remove all
  stored offline transaction data.

- set_finish_action() - to set whether after applying the offline
  transaction the system should be rebooted (default) or powered off.

Check that only one of "poweroff", or "reboot" value was passed to
set_finish_action() call.

Replace check_pending() method of the Offline interface with richer
get_status() method. The new method returns two values:
- boolean whether there is a dnf5 offline transaction scheduled for the
  next reboot
- a string->variant dictionary with the status of the dnf5 offline
  transaction