This commit expands the `Step` structure used with GitHub Actions workflow runs to include fields indicating when steps start and complete.
This information is already provided by the GitHub API, so this only involves expanding the structure, fields exported, and the associated tests.
In the future, I could see `gh` including the duration calculation which is used when viewing workflow or workflow run.
This switches `run list` and `run view` commands to measure how long ago did the run happen by using `run_started_at`, which is the timestamp of the latest run in a series, instead of `created_at`, which is the timestamp of the first run. This change also fixes accurate duration for in-progress runs.
