feat(helm): add namespaceOverride support to gateway-helm chart

[honarkhah]

What type of PR is this?

feat(helm): add namespaceOverride support to gateway-helm chart

What this PR does / why we need it:

Adds a namespaceOverride value to the gateway-helm chart, allowing users to override the namespace for all resources deployed by the chart. This is a common Helm pattern (similar to the existing nameOverride / fullnameOverride) that is especially useful when the chart is used as a sub-chart in an umbrella chart, where the release namespace may differ from the desired deployment namespace.

A new eg.namespace template helper was introduced in _helpers.tpl that returns namespaceOverride if set, falling back to .Release.Namespace. All 15 template files that previously referenced .Release.Namespace directly now use this helper instead.

Changes:

• Added namespaceOverride: "" to values.tmpl.yaml
• Added eg.namespace helper to _helpers.tpl
• Updated all templates (certgen-rbac.yaml, certgen.yaml, envoy-gateway-config.yaml, envoy-gateway-deployment.yaml, envoy-gateway-hpa.yaml, envoy-gateway-poddisruptionbudget.yaml, envoy-gateway-rbac.yaml, envoy-gateway-service.yaml, envoy-gateway-serviceaccount.yaml, infra-manager-rbac.yaml, leader-election-rbac.yaml, namespace.yaml, namespaced-infra-manager-rbac.yaml, envoy-proxy-topology-injector-webhook.yaml, NOTES.txt) to use {{ include "eg.namespace" . }} instead of {{ .Release.Namespace }}

Which issue(s) this PR fixes:
Fixes #8280

Release Notes: Yes

[netlify]

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Name	Link
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[cnvergence]

Thanks @honarkhah, maybe let's add the helm test with namespaceOverride in https://github.com/envoyproxy/gateway/tree/main/test/helm/gateway-helm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.56%. Comparing base (38b0ad1) to head (e316a56).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8281      +/-   ##
==========================================
+ Coverage   73.55%   73.56%   +0.01%     
==========================================
  Files         242      242              
  Lines       36949    36949              
==========================================
+ Hits        27178    27183       +5     
+ Misses       7851     7848       -3     
+ Partials     1920     1918       -2     

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