Add PagerDuty notification integration

[IamCharli01]

Description

Add PagerDuty as a notification channel alongside the existing Slack and Teams integrations.

Requirements:

• Support PagerDuty Events API v2
• Trigger incidents when error budget is exhausted or burn-rate alerts fire
• Resolve incidents when SLOs recover
• Add \pagerduty\ option to --notify\ CLI flag

Implementation hints:

• Add to \�zure_slo_guardian/notifiers/\
• Follow the pattern in \webhook.py\
• PagerDuty Events API v2: https://developer.pagerduty.com/api-reference/

Great for someone familiar with PagerDuty's API! 🚀