Aggregation and Reference Column Hardening

[tylerriccio33]

Summary

The purpose of this PR is to further maintain and harden how aggregations, and the reference column in particular work. There are a few spots I'd like to target in this PR

1. Briefs - the briefs were never implemented
2. The validate stub generator was a little hacky. I made it slightly more informative and robust, and included it in CI (major change)!
3. The type alias on unresolved columns for Reference was too narrow, so I expanded it to take collections.

Checklist

• I understand and agree to the Code of Conduct.
• I have followed the Style Guide for Python Code as best as possible for the submitted code.
• I have added pytest unit tests for any new functionality.