feat: add opt-in Gateway API HTTPRoute support to dify chart

[somaz94]

Adds an opt-in HTTPRoute (Gateway API) template so Dify can be exposed via a
Gateway as an alternative to Ingress. Disabled by default
(httproute.enabled: false); the existing ingress block is untouched, so current
users are unaffected. Requires the Gateway API CRDs in the cluster.

Dify fronts all components with an internal nginx proxy exposing a single
Service, so the route needs just one backendRef (the proxy Service on
service.port), mirroring the Ingress backend.

Validation (local):

• helm template: renders nothing when disabled (default), a valid HTTPRoute when enabled, and coexists with Ingress when both are enabled
• ct lint --charts charts/dify: passed
• ci/scripts/check_unused_values.py: no unused values
• helm lint: passed

I did not bump Chart.yaml (ct.yaml has check-version-increment: false and the
release workflow is maintainer-gated), so the version bump is left for the
release. Happy to trim the config surface (e.g. drop additionalRules/apiVersion)
if you prefer to keep it more minimal.

related: #398