feat: add TLS/HTTPS support for the exporter HTTP server

[edyaya]

Summary

Adds native TLS/HTTPS support for the exporter HTTP server, so /metrics and /ready endpoints can be served over HTTPS without a reverse proxy.

Configuration

YAML:

exporter:
  tlsEnabled: true
  tlsCertFilepath: /etc/kminion/tls/server.crt
  tlsKeyFilepath: /etc/kminion/tls/server.key

Environment variables: EXPORTER_TLSENABLED, EXPORTER_TLSCERTFILEPATH, EXPORTER_TLSKEYFILEPATH
Helm values: deployment.tlsEnabled, deployment.tlsCertSecret, deployment.tlsMountPath
What's included

• Config fields + validation (tlsEnabled, tlsCertFilepath, tlsKeyFilepath)
• Conditional ListenAndServeTLS in main.go
• Dockerfile: /etc/kminion/tls directory for mounting certs
• Helm chart: TLS volume/mount, httpsGet probes (deployment + daemonset), scheme: https in ServiceMonitor
  Similar to support https configuration #258, but with explicit tlsEnabled flag, config validation, and full Helm chart support.