Add namespace template variable

[asemarian]

Description of the change

• Added namespaceOverride template variable to support flexible namespace configurations.
• Introduced helper function in helpers.tpl for namespace resolution.
• Updated relevant templates to utilize the new namespace logic.
• Bumped Helm chart version to 2.21.0.

Benefits

When using cdk8s, you can't scope your resources to a namespace unless the Helm chart has a template variable for that.

Possible drawbacks

None that come to mind.

Applicable issues

N/A

Additional information

N/A

Checklist

• Chart version bumped in Chart.yaml according to semver.
• Variables are documented in the README.md

[fgeck]

Hey @dirsigler,
would it be possible to merge this PR?
I would love to use namespaceOverride

[dirsigler]

Hey @asemarian abd @fgeck ,
I think I can merge this once the Chart.yaml version bump was updated and reflects the change in README.md via helm-docs.

Thank you for your work and patience!

[dirsigler]

Please resolve conflict.