MonitorConfiguration should support disabling scheduled purgeCache [LUCENE-10675]

Monitors (using WritableQueryIndex) automatically create a private ScheduledExecutor using a hardcoded ThreadFactory to schedule a periodic call to the `purgeCache()` method.

The frequency of this periodic schedule  can be adjusted via MonitorConifguration – but it can't be disabled.

This seems odd because:
- `purgeCache()` is a public method that applications using a Monitor can invoke themselves (possible via their own ScheduledExecutor)
- In a lot of usecases there's no need for periodicly calling this method – it's enough to call exactly once after `register()`}-ing or `deleteById()`}-ing one or more batches of `MonitorQuery` objects.  there is zero reason to call it again unless/untill something changes.

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I propose we tweak the MonitorConfiguration API spec, and the use of the configuration in the code, such that if the configured purgeFrequency is a negative number, The `purgeExecutor` remains null (and no Executor is ever created)

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Migrated from [LUCENE-10675](https://issues.apache.org/jira/browse/LUCENE-10675) by Chris M. Hostetter (@hossman)

Start by reading MonitorConfiguration and the scheduled purge usage in WritableQueryIndex, then trace how purgeCache() is invoked after register() and deleteById(). Done means a negative purgeFrequency leaves purgeExecutor null and no executor is created, while existing scheduling behavior remains available for non-negative values.

Written by the indexing model from the issue text.