Add `--json` arg to `list-agents` command

[ajzobro]

Per a comment on PR #986:

we also include a --json option similar as the ones used by other commands? this is really useful (and powerful) when using alongside jq.

(https://github.com/canonical/testflinger/pull/986/changes/BASE..193a647e3ad8964d96b56fc7918ed0f222467b84#r3030798109)_

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CERTTF-964.

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