Add `--json` to `borg prune --list`

[ThomasWaldmann]

Discussed in #7021

^{Originally posted by WilliamDEdwards September 12, 2022}
I'd like to programmatically extract the archive names that Borg pruned. E.g. with a --json switch for borg prune --list (like borg list has).

--log-json is not easily parseable:

$ borg prune --log-json --list pen --keep-last 2
{"type": "log_message", "time": 1662974892.256543, "message": "Keeping archive (rule: secondly #1):     mac                                  Mon, 2022-09-12 11:25:59 [aff7f431a4a027ec328437e3d38dd72c86cc81a1ab3099b033d10f1e9e79388a]", "levelname": "INFO", "name": "borg.output.list"}

>>> import json
>>> line = '{"type": "log_message", "time": 1662974892.256543, "message": "Keeping archive (rule: secondly #1):     mac                                  Mon, 2022-09-12 11:25:59 [aff7f431a4a027ec328437e3d38dd72c86cc81a1ab3099b033d10f1e9e79388a]", "levelname": "INFO", "name": "borg.output.list"}'
>>> line = json.loads(line)
>>> line['message'].split()
['Keeping', 'archive', '(rule:', 'secondly', '#1):', 'mac', 'Mon,', '2022-09-12', '11:25:59', '[aff7f431a4a027ec328437e3d38dd72c86cc81a1ab3099b033d10f1e9e79388a]']
>>> _[5]
'mac'
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