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.

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 79.43%. Comparing base (1fe9c1b) to head (9d0cc3c).
Report is 6 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (1fe9c1b) and HEAD (9d0cc3c). Click for more details.

HEAD has 9 uploads less than BASE

Flag	BASE (1fe9c1b)	HEAD (9d0cc3c)
integration	10	1

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10795      +/-   ##
==========================================
- Coverage   89.14%   79.43%   -9.71%     
==========================================
  Files         182      183       +1     
  Lines       23297    23344      +47     
==========================================
- Hits        20768    18544    -2224     
- Misses       2529     4800    +2271     

Flag	Coverage Δ
integration	72.45% <100.00%> (-14.00%)	⬇️
unit	62.17% <100.00%> (-0.03%)	⬇️

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Components	Coverage Δ
Unit Tests	62.17% <100.00%> (-0.03%)	⬇️
Integration Tests	72.45% <100.00%> (-14.00%)	⬇️