feat: add FOLDER_ANNOTATATION logic for sidecar container in loki helm chart, making multi-tenant alerting easier

[SamuZad]

What this PR does / why we need it: The sidecar container is super convenient for adding (alerting) rules to loki.

However, as things stand, all files are mounted into the same directory, with no convenient way to separate them by tenant.

Luckily, the sidecar image already has a built-in environment variable for this, FOLDER_ANNOTATION. This PR implements the environment variable into the pod definition, in a manner consistent with other env variables within the chart.

Which issue(s) this PR fixes: Fixes #13049

Special notes for your reviewer:

Checklist

• [x] Reviewed the CONTRIBUTING.md guide (required)
• [x] Documentation added
• [x] Tests updated
• [x] 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.
• [x] Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• [x] For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• [x] 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]

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