feat(apisix): Add namespace to manifest templates

## Context

Some manifest templates in the APISIX chart do not include `namespace: {{ .Release.Namespace }}`
in their metadata, for example:
- [templates/apisix-config-cm.yml](https://github.com/apache/apisix-helm-chart/blob/master/charts/apisix/templates/apisix-config-cm.yml)
- [templates/ingress.yml](https://github.com/apache/apisix-helm-chart/blob/master/charts/apisix/templates/ingress.yaml)

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?

Start with charts/apisix/templates/apisix-config-cm.yml and charts/apisix/templates/ingress.yaml, then inspect the other manifest templates for the same metadata pattern. Render the chart with a namespace value and verify that affected resources include that namespace without breaking the manifests; the issue's requested behavior defines done.

Written by the indexing model from the issue text.