Updates to flag providers

[efahk]

This change adds.

• Passing OTEL traceparent in calls to /flags apis.
• Adds exposure event properties required for interop with experimentation
• variant splits at assignment group level.
  Probably one more PR before next release

[codecov]

Codecov Report

❌ Patch coverage is 98.86364% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.69%. Comparing base (139e398) to head (df4e5b4).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
mixpanel/flags/local_feature_flags.py	93.75%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #146      +/-   ##
==========================================
+ Coverage   93.13%   93.69%   +0.56%     
==========================================
  Files           8        9       +1     
  Lines        1151     1222      +71     
  Branches       76       81       +5     
==========================================
+ Hits         1072     1145      +73     
+ Misses         49       48       -1     
+ Partials       30       29       -1     

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[msiebert]

Looks good, just a couple comments on some tests