journal: convert --list-boots to a table

journal: convert --list-boots to a table

so it can be output as JSON as well.

```
$ build-san/journalctl --list-boots --file boot-test.journal
IDX BOOT ID                          FIRST ENTRY                 LAST ENTRY
 -3 39d66eb1925f4d01b8464d502650a714 Sat 2022-03-05 15:20:33 CET Sat 2022-03-05 16:19:21 CET
 -2 5dffeb08a27344d5ae9e2fc244bbcbc5 Fri 2022-03-11 17:23:57 CET Fri 2022-03-11 17:38:31 CET
 -1 c8ebd52915b642c39eda4bf00f864f79 Fri 2022-03-11 17:38:41 CET Fri 2022-03-11 20:03:46 CET
  0 00bcba97c7094fa88cc5d1cf2a389057 Sat 2022-03-12 20:39:08 CET Sat 2022-03-12 20:40:18 CET

$ build-san/journalctl --list-boots --file boot-test.journal -q
-3 39d66eb1925f4d01b8464d502650a714 Sat 2022-03-05 15:20:33 CET Sat 2022-03-05 16:19:21 CET
-2 5dffeb08a27344d5ae9e2fc244bbcbc5 Fri 2022-03-11 17:23:57 CET Fri 2022-03-11 17:38:31 CET
-1 c8ebd52915b642c39eda4bf00f864f79 Fri 2022-03-11 17:38:41 CET Fri 2022-03-11 20:03:46 CET
 0 00bcba97c7094fa88cc5d1cf2a389057 Sat 2022-03-12 20:39:08 CET Sat 2022-03-12 20:40:18 CET

$ build-san/journalctl --list-boots --file boot-test.journal -o json-pretty
[
        {
                "index" : -3,
                "boot_id" : "39d66eb1925f4d01b8464d502650a714",
                "first_entry" : 1646490033438495,
                "last_entry" : 1646493561047353
        },
        {
                "index" : -2,
                "boot_id" : "5dffeb08a27344d5ae9e2fc244bbcbc5",
                "first_entry" : 1647015837289036,
                "last_entry" : 1647016711595489
        },
        {
                "index" : -1,
                "boot_id" : "c8ebd52915b642c39eda4bf00f864f79",
                "first_entry" : 1647016721056382,
                "last_entry" : 1647025426397414
        },
        {
                "index" : 0,
                "boot_id" : "00bcba97c7094fa88cc5d1cf2a389057",
                "first_entry" : 1647113948506002,
                "last_entry" : 1647114018943637
        }
]
```

Resolves: #14625