feat(helm): Gateway API support; deprecate Ingress

[debsahu]

Adds Gateway API resources to the chart and marks the Ingress block as deprecated, per @sansmoraxz's review feedback on #150 about ingress-nginx retirement.

What's in

New resources

File	apiVersion	Purpose
templates/httproute.yaml	gateway.networking.k8s.io/v1	Routes the dashboard to a user-supplied Gateway.
templates/tcproute.yaml	gateway.networking.k8s.io/v1alpha2	Routes raw MQTT TCP. Requires a TCP-aware Gateway provider (Envoy Gateway, Cilium, etc.).

New gateway: values block

gateway:
  enabled: false
  parentRefs:                  # default for both routes
    - name: my-gateway
      namespace: gateway-system
  dashboard:
    enabled: true
    parentRefs: []             # override gateway.parentRefs
    hostnames: []
    matches:
      - path: { type: PathPrefix, value: / }
  mqtt:
    enabled: false
    parentRefs: []

Per-route parentRefs falls back to gateway.parentRefs when empty, so the common case is one parentRefs: entry on the chart-level block.

Ingress deprecation

• values.yaml comment marks the block DEPRECATED with the upstream retirement reference.
• values.schema.json flags the property deprecated: true.
• NOTES.txt prints a migration warning when ingress.enabled=true.
• README replaces the "Exposing MQTT externally" section with a Gateway-API-first guide; legacy Ingress is kept as a documented fallback.

The existing templates/ingress.yaml is retained for back-compat so existing users aren't broken on upgrade. Removal can land in a future major chart bump.

Verification (helm template ci .)

Path	Result
gateway.enabled=true, both routes + ingress.enabled=true	HTTPRoute (gateway.networking.k8s.io/v1) + TCPRoute (gateway.networking.k8s.io/v1alpha2) + Ingress all rendered; deprecation warning fires in NOTES
gateway.dashboard.enabled=false, gateway.mqtt.enabled=true	TCPRoute only
Default values	No gateway resources rendered (master toggle off)

helm lint . clean.

Out of scope

• Removing the legacy templates/ingress.yaml. Defer to a future major bump (e.g. chart 1.0.0) when enough time has passed for users to migrate.
• A Gateway resource. Convention is for the chart to ship Routes; users supply the Gateway via parentRefs. Bundling a Gateway would couple the chart to one provider.
• TLS configuration on the Routes themselves. For TLS at the Gateway, configure the Gateway listener; for TLS at the broker, the existing tls.* values still apply.

Refs

• Gateway API: https://gateway-api.sigs.k8s.io/
• TCPRoute guide: https://gateway-api.sigs.k8s.io/guides/tcp/
• Envoy Gateway (suggested provider): https://gateway.envoyproxy.io/

[debsahu]

Pushed f0603db to update for the dashboard-as-add-on outcome (see #151 close).

Changes from the previous push:

• gateway.dashboard.* renamed to gateway.api.* since upstream's port 8080 serves the REST API and /metrics, not a dashboard. The HTTPRoute is still useful for exposing those, just no longer dashboard-specific.
• HTTPRoute resource name: <release>-dashboard is now <release>-api.
• README and NOTES.txt updated to describe the route as "REST API + /metrics" rather than "dashboard".
• Existing service.dashboard.{type,port,nodePort} value name kept for back-compat with chart 0.1.0; it refers to the HTTP listener port regardless of what is mounted on it. Renaming that key would be a breaking change beyond this PR's scope.

Render verified for the renamed values (gateway.api.enabled and gateway.api.hostnames), TCPRoute path, and the disabled-api / TCPRoute-only path.