feat: support for multiple exporters

[thomaspoignant]

Description

This PR enable the possibility to have multiple exporter at the same time.

Under the hood we have a shared cache (with offset) that allows to store all the events in memory before publishing them to the exporters.
If for any reason the exporter is not responding we are keeping the event for the next call.

Closes issue(s)

Resolve #2505

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

[netlify]

✅ Deploy Preview for go-feature-flag-doc-preview canceled.

Name	Link
🔨 Latest commit	32f0492
🔍 Latest deploy log	https://app.netlify.com/sites/go-feature-flag-doc-preview/deploys/67cb613b65c95f0008084b99

[codecov]

Codecov Report

Attention: Patch coverage is 90.75908% with 28 lines in your changes missing coverage. Please review.

Project coverage is 85.66%. Comparing base (5caf22c) to head (32f0492).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
exporter/event_store.go	87.61%	9 Missing and 4 partials ⚠️
cmd/relayproxy/service/gofeatureflag.go	82.05%	7 Missing ⚠️
cmd/relayproxy/config/config.go	83.87%	4 Missing and 1 partial ⚠️
exporter/manager.go	93.02%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3171      +/-   ##
==========================================
+ Coverage   84.95%   85.66%   +0.71%     
==========================================
  Files         113      115       +2     
  Lines        5276     5475     +199     
==========================================
+ Hits         4482     4690     +208     
+ Misses        630      615      -15     
- Partials      164      170       +6     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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

See analysis details on SonarQube Cloud