Add jitter to accept randomization of the retrieval time

[thomaspoignant]

Description

Adding an option to avoid having true periodicity when retrieving your flags.
It is useful to avoid having spike on your flag configuration storage in case your application is starting multiple instances at the same time.

We ensure a deviation that is maximum + or - 10% of your polling interval.
Default: false

Changes include

• New feature (non-breaking change that adds functionality)

Closes issue(s)

Resolve #942

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	ee5915d
🔍 Latest deploy log	https://app.netlify.com/sites/go-feature-flag-doc-preview/deploys/64c3f006785db70008dfef13

[codecov]

Codecov Report

Merging #943 (ee5915d) into main (19286b6) will increase coverage by 0.01%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##             main     #943      +/-   ##
==========================================
+ Coverage   89.50%   89.52%   +0.01%     
==========================================
  Files          66       66              
  Lines        3002     3016      +14     
==========================================
+ Hits         2687     2700      +13     
- Misses        257      258       +1     
  Partials       58       58              

Files Changed	Coverage Δ
cmd/relayproxy/config/config.go	94.39% <ø> (ø)
cmd/relayproxy/service/gofeatureflag.go	62.17% <0.00%> (-0.33%)	⬇️
config.go	100.00% <ø> (ø)
background_updater.go	100.00% <100.00%> (ø)
feature_flag.go	96.92% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication