Allows snapshots to have a list of unique keys

[AGPapa]

resolves #9992

Problem

Currently the unique_key must be a string. It can be a column name or an expression. This PR changes it so it can be a list of column names or expressions.

Solution

This solution is modeled off of #4618.

It is paired with this change in dbt-adapters dbt-labs/dbt-adapters#182

I'm open to feedback! Particularly on the best way to add tests for this feature. I've run this in development on one of my dbt projects and it works as expected.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.68%. Comparing base (84456f5) to head (cffc43d).
Report is 189 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9993      +/-   ##
==========================================
- Coverage   88.74%   88.68%   -0.06%     
==========================================
  Files         180      180              
  Lines       22449    22448       -1     
==========================================
- Hits        19923    19909      -14     
- Misses       2526     2539      +13     

Flag	Coverage Δ
integration	85.96% <100.00%> (-0.18%)	⬇️
unit	63.37% <100.00%> (-0.01%)	⬇️

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