Add default averager to support general trailing aggregates #11387

[yuanlihan]

Fixes #11387.

Description

Add a default type Averager to support general trailing aggregates using existing Druid aggregators.

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Key changed/added classes in this PR

• BaseAverager
• MovingAverageIterable
• DefaultAggregateAverager
• DefaultAggregateAveragerFactory

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This PR has:

• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[marcospassos]

Same use case here.

[stale]

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