feat: Tracking exporter

[thomaspoignant]

Description

In this PR we introduce a new type of exporter that stored what we called trackingEvents.
Those events are here to be able to collect data about the behavior of your application linked to a feature flag.
You can read the OpenFeature Spec about this concept: https://openfeature.dev/docs/reference/concepts/tracking/.

As a follow-up of this, we will start implementing feature events in the different OpenFeature providers for GO Feature Flag.

Checklist

• I have tested this code
• I have added unit test to cover this code
• I have updated the documentation (README.md and /website/docs)
• I have followed the contributing guide

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[codecov]

Codecov Report

Attention: Patch coverage is 81.22744% with 52 lines in your changes missing coverage. Please review.

Project coverage is 84.25%. Comparing base (cfa26b0) to head (a0f5a13).

Files with missing lines	Patch %	Lines
exporter/fileexporter/exporter.go	44.89%	24 Missing and 3 partials ⚠️
ffcontext/context.go	46.66%	8 Missing ⚠️
feature_flag.go	93.81%	4 Missing and 2 partials ⚠️
tracking.go	72.22%	4 Missing and 1 partial ⚠️
exporter/feature_event.go	89.28%	2 Missing and 1 partial ⚠️
exporter/tracking_event.go	86.95%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3176      +/-   ##
==========================================
- Coverage   84.78%   84.25%   -0.53%     
==========================================
  Files         122      124       +2     
  Lines        5706     5851     +145     
==========================================
+ Hits         4838     4930      +92     
- Misses        689      737      +48     
- Partials      179      184       +5     

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
1 Accepted issue

Measures
0 Security Hotspots
0.0% Coverage on New Code
1.3% Duplication on New Code

See analysis details on SonarQube Cloud