Add skew and kurtosis operations

[jbrooks-stripe]

Summary

Adds operations for skew and kurtosis.

The implementation uses the same Welford/Knuth approach as the variance operation. https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Higher-order_statistics

I have the operations set as non-deletable as the reversals weren't straightforward (variance is also set this way).

Why / Goal

We have existing features in our previous feature store which rely on these statistics.

Test Plan

• [x] Added Unit Tests
• [x] Covered by existing CI
• [x] Integration tested

Checklist

• [x] Documentation update

Reviewers

[nikhilsimha]

Solid PR!