Add retention days to fly vol snapshots ls

[tvdfly]

Change Summary

Each snapshot may have a different retention value, because we do not currently update the retention on existing snapshots when the snapshot retention setting is changed on the volume. This change exposes the retention value for each snapshot.

Depends on: https://github.com/superfly/fly-go/pull/55

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Documentation

• [ ] Fresh Produce
• [ ] In superfly/docs, or asked for help from docs team
• [x] n/a