feat: Add Kafka credentials support via Kubernetes secrets

[Biowoolf]

This PR adds support for external Kafka credentials through Kubernetes secrets, improving security by avoiding hardcoded credentials in configuration files.

Changes:

• Add SecretRef field to MilvusKafka struct
• Add getKafkaCredentials function to retrieve credentials from secret
• Update updateConfigMap to inject Kafka credentials automatically
• Add example configuration and documentation

Usage:

Users can now reference a Kubernetes secret containing Kafka credentials:

dependencies:
  kafka:
    external: true
    secretRef: "kafka-credentials"

This follows the same pattern as MinIO credentials and is fully backward compatible.

[sre-ci-robot]

Welcome @Biowoolf! It looks like this is your first PR to zilliztech/milvus-operator 🎉

[Biowoolf]

/assign @LoveEachDay

[codecov]

Codecov Report

❌ Patch coverage is 5.55556% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.39%. Comparing base (6f4b755) to head (6cb3c8f).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/controllers/configmaps.go	5.55%	16 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #377      +/-   ##
==========================================
- Coverage   75.57%   75.39%   -0.18%     
==========================================
  Files          65       65              
  Lines        7070     7088      +18     
==========================================
+ Hits         5343     5344       +1     
- Misses       1502     1518      +16     
- Partials      225      226       +1     

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[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[Biowoolf]

what do i need to do for this PR to be merged?

[haorenfsa]

Thank you @Biowoolf. I'm sorry for the delay at my side.

You need to run make generate-all to generate the updates for the CRD & helm chart templates. The rest looks good.