Add bounded unique count aggregation

[jbrooks-stripe]

Summary

Adds a BOUNDED_UNIQUE_COUNT aggregation. This will allow exact unique/distinct counts, but will cap at a given value to keep memory usage constant.

Why / Goal

We have use cases where we'd prefer an exact solution instead of the approx equivalents, but want to have protections in place so that memory doesn't become an issue.

Test Plan

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

Checklist

• [x] Documentation update

Reviewers