Support multiple unique keys in snapshots

[gshank]

Resolves #9992

Problem

Users may want to snapshot tables with multiple unique keys.

Solution

Redefine the SnapshotConfig unique_key attribute as a Optional[Union[str, List[str]]]. Supporting code in dbt-adapters in dbt-labs/dbt-adapters#326.

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 or 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.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.13%. Comparing base (3d96b4e) to head (885d9b4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10795      +/-   ##
==========================================
- Coverage   89.15%   89.13%   -0.02%     
==========================================
  Files         183      183              
  Lines       23443    23443              
==========================================
- Hits        20901    20897       -4     
- Misses       2542     2546       +4     

Flag	Coverage Δ
integration	86.42% <100.00%> (-0.02%)	⬇️
unit	62.12% <100.00%> (ø)

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Components	Coverage Δ
Unit Tests	62.12% <100.00%> (ø)
Integration Tests	86.42% <100.00%> (-0.02%)	⬇️