feat(apisix): Add namespace to manifest templates

[vramperez]

Context

Some manifest templates in the APISIX chart do not include namespace: {{ .Release.Namespace }}
in their metadata, for example:

• templates/apisix-config-cm.yml
• templates/ingress.yml

This is inconsistent with Helm best practices, where templates should respect
the namespace provided during installation via --namespace or .Release.Namespace.

Expected behavior

All manifest templates should include:

metadata:
  namespace: {{ .Release.Namespace }}

Is this an intentional design decision?

If not, would a PR adding namespace: {{ .Release.Namespace }} to the affected
templates be welcome?