Add start_time and end_time to 'data-updates' output

[gklei]

Closes #94

[ashfulcra]

Thanks for the quick turnaround on #94 — this covers the ergonomics well.

One nuance worth raising before it merges, as a question rather than an objection.

The values echoed here are data_updates' own start_time/end_time parameters — what the CLI was called with. So a client reading the output still can't distinguish "the server covered the whole interval I asked for" from a server-side truncation: both produce output showing the interval I requested.

That matters for the use case in #94 specifically. To advance a sync checkpoint safely, a client needs the interval the server actually covered, attested in the response body — otherwise moving the cursor to end_time risks skipping records the server silently didn't include.

Worth noting the shape here already accommodates that: since **resp is spread last, a server-provided start_time/end_time would take precedence over the echoed pair. So this doesn't need to change to become server-attested later — it just needs something populating those keys server-side.

Either way the echo is useful on its own, and we're glad to have it.

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