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

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)_

Start at the list-agents command and compare its argument handling with the other commands that already support --json, using PR #986 for context. Done means list-agents accepts --json and produces JSON suitable for jq, with the relevant command behavior covered by the project's existing checks.

Written by the indexing model from the issue text.