feat: DA feature flags

[anchenyi]

https://msazure.visualstudio.com/Microsoft%20Teams%20Extensibility/_workitems/edit/31619041

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.31%. Comparing base (8553bd7) to head (bd26f67).
Report is 15 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev   #13325   +/-   ##
=======================================
  Coverage   90.31%   90.31%           
=======================================
  Files         611      610    -1     
  Lines       36361    36363    +2     
  Branches     7180     7184    +4     
=======================================
+ Hits        32841    32843    +2     
  Misses       1524     1524           
  Partials     1996     1996           

Files with missing lines	Coverage Δ
packages/fx-core/src/common/featureFlags.ts	96.87% <100.00%> (+0.20%)	⬆️
.../src/component/driver/teamsApp/createAppPackage.ts	94.90% <100.00%> (-0.37%)	⬇️
...kages/fx-core/src/component/generator/generator.ts	86.86% <100.00%> (+0.27%)	⬆️
...omponent/generator/templates/templateReplaceMap.ts	98.11% <100.00%> (+0.07%)	⬆️
packages/fx-core/src/core/FxCore.ts	89.43% <100.00%> (+0.02%)	⬆️
packages/fx-core/src/question/constants.ts	96.61% <100.00%> (+0.24%)	⬆️
...x-core/src/question/options/AddKnowledgeOptions.ts	100.00% <ø> (ø)
...s/vscode-extension/src/treeview/treeViewManager.ts	90.84% <100.00%> (ø)

... and 8 files with indirect coverage changes

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