feat: Add ElastAlert2 Provider

[aliyevr889]

Closes #4232

Summary

Adds a new ElastAlert2 Provider for Keep, integrating with the ElastAlert2 alerting framework for Elasticsearch.

Features

• Webhook support — receive real-time alerts from ElastAlert2 via its http_post2 alerter
• Pull rule error status from ElastAlert2 REST API
• Automatic severity mapping: critical/error/warning/info/debug + numeric priority levels (1-4)
• Full timestamp parsing from @timestamp and other Elasticsearch date formats

Files Added

• keep/providers/elastalert2_provider/__init__.py
• keep/providers/elastalert2_provider/elastalert2_provider.py
• keep/providers/elastalert2_provider/README.md (includes rule configuration examples)

Webhook Setup

Add to any ElastAlert2 rule YAML:

alert:
  - post2
http_post_url: "https://<your-keep-url>/alerts/event/elastalert2"
http_post_payload:
  alert_name: "%(rule_name)s"
  message: "%(message)s"
  severity: "warning"

/bounty $50 
/claim #4232

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[shahargl]

AI slop