refactor: redesign office chat feature flag

[GavinGu07]

https://msazure.visualstudio.com/Microsoft%20Teams%20Extensibility/_workitems/edit/29400043/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.60%. Comparing base (6744ee8) to head (6dcd915).
Report is 257 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev   #12371   +/-   ##
=======================================
  Coverage   94.60%   94.60%           
=======================================
  Files         582      582           
  Lines       34484    34486    +2     
  Branches     6680     6748   +68     
=======================================
+ Hits        32624    32626    +2     
  Misses       1586     1586           
  Partials      274      274           

Files with missing lines	Coverage Δ
packages/fx-core/src/common/featureFlags.ts	100.00% <100.00%> (ø)