restrict behavior flags to firing a warning only the first time

[mikealfare]

resolves dbt-labs/dbt-redshift#915

Description

We're firing a warning per model for behavior changes. We should fire a warning per run.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.37%. Comparing base (7e2edee) to head (069190c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
+ Coverage   67.35%   67.37%   +0.01%     
==========================================
  Files          52       52              
  Lines        3370     3372       +2     
==========================================
+ Hits         2270     2272       +2     
  Misses       1100     1100              

Flag	Coverage Δ
unit	67.37% <100.00%> (+0.01%)	⬆️

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[mikealfare]

I've verified locally that the warning only shows once in the logs for the dbt-redshift flag, but I'm having a tough time grabbing the event manager in the testing setup to verify it programmatically.

[mikealfare]

Got it: dbt-labs/dbt-redshift#916