[#12092] Refactor FeatureFlagService

[kojandy]

Related issue: #12092.

[codecov]

Codecov Report

Attention: Patch coverage is 67.74194% with 10 lines in your changes missing coverage. Please review.

Project coverage is 33.76%. Comparing base (fb2d834) to head (752c63e).

Files with missing lines	Patch %	Lines
...eflag/provider/FeatureFlagServiceProviderImpl.java	61.11%	3 Missing and 4 partials ⚠️
.../featureflag/service/SimpleFeatureFlagService.java	80.00%	1 Missing and 1 partial ⚠️
...npoint/featureflag/service/FeatureFlagService.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #12102      +/-   ##
============================================
- Coverage     33.78%   33.76%   -0.02%     
+ Complexity    10542    10537       -5     
============================================
  Files          3787     3789       +2     
  Lines         89988    89997       +9     
  Branches       9573     9572       -1     
============================================
- Hits          30399    30391       -8     
- Misses        56967    56981      +14     
- Partials       2622     2625       +3     

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