Add --json, --jq, and --template flags to `gh agent-task list`,
consistent with the pattern used by `gh pr list --json`,
`gh issue list --json`, etc.
This implements the ExportData interface on capi.Session and defines
SessionFields for the available JSON fields:
id, name, status, repository, createdAt, updatedAt,
pullRequestNumber, pullRequestUrl.
Closes https://github.com/cli/cli/issues/12805 (partial)
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>