feat: add message about unconfigured feature flags when user runs cdk synth

[vivian12300]

Notify users about the number of unconfigured feature flags in their application and the cdk flags command when they run cdk synth.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.32%. Comparing base (c6e6c46) to head (ea90145).

Files with missing lines	Patch %	Lines
packages/aws-cdk/lib/cli/cdk-toolkit.ts	78.57%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #765      +/-   ##
==========================================
+ Coverage   81.21%   81.32%   +0.11%     
==========================================
  Files          61       62       +1     
  Lines        8267     8284      +17     
  Branches      949      955       +6     
==========================================
+ Hits         6714     6737      +23     
+ Misses       1528     1522       -6     
  Partials       25       25              

Flag	Coverage Δ
suite.unit	81.32% <84.21%> (+0.11%)	⬆️

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