Create GitLab issues from PagerDuty webhook payload (Part 1)

What does this MR do?

Contributes to #227202 (closed)

Creates the requires classes to parse PagerDuty webhook payload, and create GitLab incident issue based on the incident details.

The MR is getting big, so the second part will implement service class connecting the Webhook controller with these classes.

The implementation is behind pagerduty_webhook feature flag

Screenshots

Issue example

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team