support continuation / resume of runs

[leondz]

It should be possible to continue an incomplete run, given the partial report.jsonl.

This may involve:

• reading & loading the config from the report.jsonl
• working out which attempts are complete and which need to be processed still (based on attempt status)
• re-creating the complete and, maybe? incomplete attempts
• setting up a run using the remaining, non-executed probes
• writing a new report.jsonl including what's known so far
• running those not-yet-executed probes

The interface might look like:

• --continue switch: take json from report, process attempts, continue from where we stopped. requires robust config storage

[leondz]

issue #833 contributed to this

[fcanogab]

I've contributed an initial proposal to implement this functionality in #1107.
It's my first contribution to Garak and probably I'm missing important bits, so I'd appreciate your feedback to improve the PR.

[EricXQiu]

If we can resume runs this is great. Looking forward to this feature.

[github-actions]

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