chore(helm): Add Memcached StatefulSet PVC labels to Helm Chart values

[sousa-andre]

What this PR does / why we need it:
This change allows custom labels to be added to the PersistentVolumeClaims for the chunks and results cache created by their StatefulSet. This is useful, among other things, to prevent alerts from being triggered when the disk becomes full.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[CLAassistant]

All committers have signed the CLA.