Expose namespace aggregation duration via API

What does this MR do and why?

In !120988 (merged), an application setting was added to control the lease lock we use for the Namespace::AggregationSchedule updates.

This MR exposes that field (namespace_aggregation_schedule_lease_duration_in_seconds) for updating via the REST API. This will allow QA e2e tests to adjust the setting.

For the same reason, I've not added a changelog trailer, but happy to add one if necessary.

But why no documentation, vij?

The setting is currently only used behind a feature flag, so I'll add a note to the rollout issue (#408463 (closed)) to ensure documentation gets added for the API when the FF is removed.

Refs #416569 (comment 1475230759)

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