feat: Add config to allow surpressing notification on launch (flag cache load)

[8ma10s]

Description

This pull request introduces a new feature to disable notifications on initialization and refactors the caching mechanism to support this feature. It also includes updates to the test suite to ensure the new functionality works as expected.

Problem

• go-feature-flag always sent notification of diff to all Notifierson ffClient initialization because it detects the difference between the initial cache (= always empty) and the flags retrieved.

Resolution

• introduce DisableNotificationOnInit configuration key. If set to true, avoid sending the notification on cache initialization (it will still send notification when the change is detected AFTER initialization)
  • Since the behavior (should) stays the same if the new config is NOT set, it should not introduce any breaking change.

How to test

• I've added some tests to make sure the behavior stays the same when the new config is not explicitly set. You can run go test ./...
• Or, simply initialize ffClient should do the trick too.

Closes issue(s)

Resolve #2532

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.

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[codecov]

Codecov Report

Attention: Patch coverage is 86.20690% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.90%. Comparing base (9ce4b93) to head (fa5dda7).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
feature_flag.go	86.36%	2 Missing and 1 partial ⚠️
cmd/relayproxy/service/gofeatureflag.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2534      +/-   ##
==========================================
+ Coverage   85.84%   85.90%   +0.06%     
==========================================
  Files         100      100              
  Lines        3554     3577      +23     
==========================================
+ Hits         3051     3073      +22     
- Misses        378      379       +1     
  Partials      125      125              

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[sonarcloud]

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 SonarCloud