[#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 33.70%. Comparing base (ad5aea8) to head (cde8a53).

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     33.70%   33.70%   -0.01%     
+ Complexity    10565    10561       -4     
============================================
  Files          3808     3810       +2     
  Lines         90385    90394       +9     
  Branches       9587     9586       -1     
============================================
+ Hits          30467    30469       +2     
- Misses        57288    57290       +2     
- Partials       2630     2635       +5     

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