bitnami/thanos template extraFlags in sharded statefulset storegateway like the template for non-sharded statefulset storegateway

[TomHellier]

Description of the change

The sharded storegateway should have as similar behaviour to the non-sharded store gateway.

Follows the same pattern as the non-sharded store gateway here.

Benefits

Easier configuration of extraFlags

Possible drawbacks

If users set relabel-configs as part of the extraFlags, then it will overwrite the relabel-configs written by the sharded storegateway. But, I'd imagine anyone who is doing that would understand how to resolve that if they're opting into the non standard sharded configuration

Applicable issues

• fixes Thanos helm chart failing to tpl extraFlags for sharded storegateway statefulsets #36359

Additional information

Checklist

• Chart version bumped in Chart.yaml according to semver. This is not necessary when the changes only affect README.md files.
• Variables are documented in the values.yaml and added to the README.md using readme-generator-for-helm
• Title of the pull request follows this pattern [bitnami/<name_of_the_chart>] Descriptive title
• All commits signed off and in agreement of Developer Certificate of Origin (DCO)