Adding Alerting Comments system indices

[toepkerd]

Description

Adding Alerting Comments system indices to the Alerting Plugin's System Index Descriptors.

This is a followup to https://github.com/opensearch-project/alerting/pull/1655

TODO: need to add IT to ensure users without permission can't access system indices

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] API changes companion pull request created.
• [ ] Commits are signed per the DCO using --signoff.
• [ ] Public documentation issue/PR created.

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