Add support for Helm namespace override

[jgmartinez]

What this PR does

Add support for Namespace Override for Helm. This is useful to override Mimir's namespace when running multichart helm installations

Checklist

• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].

[CLAassistant]

All committers have signed the CLA.

[dimitarvdimitrov]

doesn't the helm --namespace flag do this job? For example helm upgrade --namespace MY_NAMESPACE mimir grafana/mimir-distributed

[chencs]

The CHANGELOG has just been cut to prepare for the next release. Please rebase main and eventually move the CHANGELOG entry added / updated in this PR to the top of the operations/helm/charts/mimir-distributed/CHANGELOG.md document. Thanks!