Add feature flags

[vigneshhari]

This implementation adds the concept of a feature flag and how it can be used in API's

[shivankacker]

@vigneshhari this is under progress right? I would like to go ahead with the frontend implementation for this. I am imagining there will be an API exposed that returns whether the particular user is allowed to use a feature or not, which we can use in the frontend to enable/disable features.

[vigneshhari]

@sainak is taking over this MR

[codecov]

Codecov Report

Attention: Patch coverage is 94.30380% with 9 lines in your changes missing coverage. Please review.

Project coverage is 64.91%. Comparing base (a06913e) to head (89952e2).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
care/utils/registries/feature_flag.py	91.48%	4 Missing ⚠️
care/facility/admin.py	90.00%	0 Missing and 1 partial ⚠️
care/facility/models/facility_flag.py	95.65%	1 Missing ⚠️
care/users/admin.py	91.66%	0 Missing and 1 partial ⚠️
care/users/models.py	95.83%	1 Missing ⚠️
care/utils/models/base.py	96.87%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2429      +/-   ##
===========================================
+ Coverage    64.57%   64.91%   +0.33%     
===========================================
  Files          240      242       +2     
  Lines        13646    13801     +155     
  Branches      1950     1975      +25     
===========================================
+ Hits          8812     8959     +147     
- Misses        4460     4466       +6     
- Partials       374      376       +2     

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