Add round support for DS-HLL

[ccaominh]

Description

Since the Cardinality aggregator has a "round" option to round off estimated
values generated from the HyperLogLog algorithm, add the same "round" option to
the DataSketches HLL Sketch module aggregators to be consistent.

---

This PR has:

• been self-reviewed.
• added documentation for new or modified features or behaviors.
• added unit tests or modified existing tests to cover new code paths.

---

For reviewers: the key changed/added classes in this PR are HllSketchAggregatorFactory, HllSketchBuildAggregatorFactory, HllSketchMergeAggregatorFactory, and HllSketchAggregatorTest.

[gianm]

The similar patch for hyperUnique and cardinality aggregators is here: #4720

[gianm]

The test failure looks legitimate. It happened while running in standards-compliant null handling mode, which I suppose changed the results a bit (2 rows came back instead of 1).

Failed tests: 
org.apache.druid.query.aggregation.datasketches.hll.sql.HllSketchSqlAggregatorTest.testApproxCountDistinctHllSketchIsRounded(org.apache.druid.query.aggregation.datasketches.hll.sql.HllSketchSqlAggregatorTest)
  Run 1: HllSketchSqlAggregatorTest.testApproxCountDistinctHllSketchIsRounded:419 expected:<1> but was:<2>
  Run 2: HllSketchSqlAggregatorTest.testApproxCountDistinctHllSketchIsRounded:419 expected:<1> but was:<2>
  Run 3: HllSketchSqlAggregatorTest.testApproxCountDistinctHllSketchIsRounded:419 expected:<1> but was:<2>
  Run 4: HllSketchSqlAggregatorTest.testApproxCountDistinctHllSketchIsRounded:419 expected:<1> but was:<2>

[gianm]

By the way, the mode is controlled by a system property. You can set -Ddruid.generic.useDefaultValueForNull=false locally to get the same behavior in your IDE or local Maven runs.

[gianm]

This LGTM after the adjustment to the tests to handle the other null handling mode.

[gianm]

LGTM - thanks @ccaominh!