[NU-1921] Add standard deviation and variance aggregations

[paw787878]

Describe your changes

Checklist before merge

• Related issue ID is placed at the beginning of PR title in [brackets] (can be GH issue or Nu Jira issue)
• Code is cleaned from temporary changes and commented out lines
• Parts of the code that are not easy to understand are documented in the code
• Changes are covered by automated tests
• Showcase in dev-application.conf added to demonstrate the feature
• Documentation added or updated
• Added entry in Changelog.md describing the change from the perspective of a public distribution user
• Added MigrationGuide.md entry in the appropriate subcategory if introducing a breaking change
• Verify that PR will be squashed during merge

[github-actions]

created: #7322
⚠️ Be careful! Snapshot changes are not necessarily the cause of the error. Check the logs.

[jedrz]

Please add changelog entry.

[jedrz]

For me it would be clearer to represent this class as 3 cases to avoid null handling:

• empty sum
• double sum
• big decimal sum

The problem is, I don't know if Flink can serialize such class hierarchy efficiently.

[paw787878]

Please add changelog entry.
done

[jedrz]

We can remove ifs above and do sth like:

case PopulationVariance => populationVariance
case SampleVariance => sampleVariance(populationVariance)
case SampleStandardDeviation => MathUtils.largeFloatSqrt(populationVariance)
case PopulationStandardDeviation =>  MathUtils.largeFloatSqrt(sampleVariance(populationVariance))

?