Add Gateway API (HTTPRoute) support due to ingress-nginx retirement

[gagantrivedi]

Background

Kubernetes ingress-nginx controller is being retired with maintenance ending in March 2026, with no further security updates after that date:
https://www.kubernetes.dev/blog/2025/11/12/ingress-nginx-retirement/

The Kubernetes SIG Network recommends migrating to Gateway API, which is now the preferred way to handle ingress traffic.

Current Limitation

The Flagsmith Helm chart currently only supports standard Kubernetes Ingress resources (networking.k8s.io/v1). There is no support for Gateway API resources (gateway.networking.k8s.io/v1).

Requested Feature

Add support for Kubernetes Gateway API alongside existing Ingress support:

• Support for HTTPRoute resources for frontend, api, and sse endpoints
• Allow users to choose between Ingress (legacy) and Gateway API (modern) via values.yaml
• Maintain backward compatibility with existing Ingress configurations

Example configuration structure:

gateway:
  enabled: false  # New option
  className: null
  frontend:
    enabled: false
    parentRefs: []
    hostnames: []
    rules: []
  api:
    enabled: false
    parentRefs: []
    hostnames: []
    rules: []
  sse:
    enabled: false
    parentRefs: []
    hostnames: []
    rules: []

# Existing ingress config remains for backward compatibility
ingress:
  frontend:
    enabled: false
    # ... existing fields



References

- https://gateway-api.sigs.k8s.io/
- https://www.kubernetes.dev/blog/2025/11/12/ingress-nginx-retirement/
- https://gateway-api.sigs.k8s.io/api-types/httproute/