feat: add k8s secret retriever

[ndrshrzg]

Description

Retrieving feature flags from Kubernetes ConfigMap is already supported. Support for Kubernetes Secrets is missing.

This PR adds a k8sretriever.SecretRetriever implementation of the Retriever interface. Also adds corresponding unit test.

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

❌ Patch coverage is 66.66667% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.81%. Comparing base (3ba39a0) to head (eb5dd36).

Files with missing lines	Patch %	Lines
cmdhelpers/retrieverconf/init/retriever_init.go	0.00%	11 Missing ⚠️
retriever/k8sretriever/secret_retriever.go	90.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3986      +/-   ##
==========================================
- Coverage   83.92%   83.81%   -0.11%     
==========================================
  Files         137      138       +1     
  Lines        6813     6854      +41     
==========================================
+ Hits         5718     5745      +27     
- Misses        884      898      +14     
  Partials      211      211              

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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

[thomaspoignant]

Hey thanks for opening this pull request.
I have a question, in which case do you think using a secret is better than using configmaps? I can't see the value of storing feature flags inside secrets.

Do you have any use case where you thing this is better to have secrets instead of confimaps?