[stable/prometheus-node-exporter] Add support for namespace specification

[arruzk]

What this PR does / why we need it:
It adds a namespaceOverride parameter, which allows this chart to be used in cases where templating is used across namespaces. By providing this parameter, the resulting templating will output to the specified namespace.

Special notes for your reviewer:
This change is needed for implementing #18837 and inspired by #15202

Checklist

• DCO signed
• Chart Version bumped
• Variables are documented in the README.md
• Title of the PR starts with chart name (e.g. [stable/chart])

Mentioning the fine people taking care of this chart: @vsliouniaev @gianrubio

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: arruzk
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[k8s-ci-robot]

Hi @arruzk. Thanks for your PR.

I'm waiting for a helm member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[arruzk]

/assign @vsliouniaev