EKSA Curated Packages : Alert Manager for Prometheus

**What would you like to be added**:

As an Admin for EKSA Cluster, I want to monitor the health of critical components on cluster like health metrics, certificate expiry date metric and route alerts to pager duty or e-mail.

https://github.com/prometheus/alertmanager

**Why is this needed**:
- Monitoring and Alerting for Production cluster deployment use case

Start by reviewing the linked Prometheus Alertmanager project and the repository's conventions for EKSA curated packages. No files or tests are identified in the issue, so first locate the relevant package entry points and existing monitoring integrations. Done means production clusters can monitor critical health and certificate-expiry metrics and route alerts to PagerDuty or email.

Written by the indexing model from the issue text.