feat(helm): add support for custom alert rule annotations

[hamedsalim1999]

feat(helm): add support for custom alert rule annotations

What this PR does / why we need it:
This PR introduces support for adding custom annotations to Loki Helm chart alerting rules, similar to how additionalRuleLabels already works.

• Adds a new monitoring.rules.additionalRuleAnnotations values key.
• These annotations are merged into the annotations: block of each built-in alert in alerts.yaml.tpl.

This enables users to enrich alerts with metadata such as runbook_url, summary, or any other contextual information without having to fork or manually patch the chart.

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Which issue(s) this PR fixes:
Fixes #<19192>

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Special notes for your reviewer:

• The change mirrors the existing pattern for additionalRuleLabels.
• Backward compatible: if additionalRuleAnnotations is not defined, behavior is unchanged.
• Example usage in values.yaml:

monitoring:
  rules:
    additionalRuleAnnotations:
      runbook_url: "https://runbooks.example.com/oncall/loki"
      summary: "Investigate upstream errors; see runbook."

This will render in each alert’s annotations: block:

annotations:
  message: "...existing alert message..."
  runbook_url: "https://runbooks.example.com/oncall/loki"
  summary: "Investigate upstream errors; see runbook."

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Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added (values.yaml and chart README updated)
• Tests updated (if applicable for chart validation)
• Title matches the required conventional commits format

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