Add Gateway API HTTPRoute support to the Helm chart (#10095)

Add an opt-in Gateway API HTTPRoute template to the Helm chart as an
alternative to the existing Ingress, addressing #9942. It is disabled by
default (httpRoute.enabled: false) so existing installs are unaffected.

The template mirrors the existing ingress.yaml conventions: the
pgadmin4.fullname backend, commonLabels/commonAnnotations propagation, and
a hostname that falls back to ingress.hostname when httpRoute.hostnames is
unset. parentRefs is required (a fail guard fires when it is empty),
apiVersion defaults to gateway.networking.k8s.io/v1, and the default rule
forwards "/" (PathPrefix) to the pgAdmin service. Custom hostnames and
rules can be supplied for full control. The new values are documented in
the chart README.