Optional Backup Summary to the log

Hi, please provide an option to add the useful Backup Summary also to the logs.

The btrbk client prints the summary like this:
```
--------------------------------------------------------------------------------
Backup Summary (btrbk command line client, version 0.33.0-dev)

Date: Thu Sep 28 07:35:16 2023
Config: /etc/btrbk/btrbk.conf
Filter: all

Legend:
=== up-to-date subvolume (source snapshot)
+++ created subvolume (source snapshot)
--- deleted subvolume
*** received subvolume (non-incremental)
>>> received subvolume (incremental)
--------------------------------------------------------------------------------
/mnt/btrfs-top-lvl/volumes/root
+++ /mnt/btrfs-top-lvl/snapshots/root/root.20230928T073516+0200
>>> /mnt/local/data/snapshots/root/root.20230928T073516+0200
>>> /mnt/usb/mobile/snapshots/desktop_one/root/root.20230928T073516+0200

/mnt/btrfs-top-lvl/volumes/home
+++ /mnt/btrfs-top-lvl/snapshots/home/home.20230928T073516+0200
>>> /mnt/local/data/snapshots/home/home.20230928T073516+0200
>>> /mnt/usb/mobile/snapshots/desktop_one/home/home.20230928T073516+0200
...
```