feat: Allow deactivating polling by setting a negative polling

[thomaspoignant]

Description

In this PR we are allowing a negative value for the config pollingInterval duration.
When you set a negative value it will deactivate the polling completely. You can always use the /admin/v1/retriever/refresh endpoint if you are using the relay-proxy or the ForceRefresh() function if you are using the go module.

Closes issue(s)

Resolve #3203

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	405bb59
🔍 Latest deploy log	https://app.netlify.com/sites/go-feature-flag-doc-preview/deploys/67d2aa3a15d24c00082eac01

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.79%. Comparing base (dd1f8ab) to head (405bb59).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3205      +/-   ##
==========================================
- Coverage   85.79%   85.79%   -0.01%     
==========================================
  Files         121      121              
  Lines        5668     5666       -2     
==========================================
- Hits         4863     4861       -2     
  Misses        629      629              
  Partials      176      176              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[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