Validate PagerDuty payload using json_schemer

What does this MR do?

Closes #229125 (closed)

Validates PagerDuty Webhook Payload using json_schemer.

Read more about json_schemer vs json-schema here.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~
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