feat: add namespaceOverride support for sub-chart deployments

[ElayGelbart]

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• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?

Changes

Added namespaceOverride support to allow deploying Flagsmith to a specific namespace when used as a sub-chart.

Closes #452

• Added flagsmith.namespace helper in _helpers.tpl that returns namespaceOverride if set, otherwise falls back to .Release.Namespace
• Updated all resource templates to use {{ include "flagsmith.namespace" . }} for the namespace field

How did you test this code?

1. Added the Flagsmith chart as a dependency in a parent chart:

   # Chart.yaml
   dependencies:
     - name: flagsmith
       version: "x.x.x"
       repository: "file://../flagsmith"

2. Set the namespace override in the parent chart's values:

   # values.yaml
   flagsmith:
     namespaceOverride: "flagsmith-ns"

3. Ran helm template and verified all Flagsmith resources have namespace: flagsmith-ns in their metadata

4. Deployed to minikube and verified all resources are created in the correct namespace and functioning properly

5. Verified default behavior (without namespaceOverride) still uses the release namespace

[ElayGelbart]

@khvn26

[khvn26]

Hi @ElayGelbart, thanks for your contribution, and for the nudge.

Is overriding namespaces like this a common pattern in Helm world? I was under the impression that additional templating/orchestration is used when users need to deploy to multiple namespaces, e.g. Helmfile.