Implement payload field extract service and model

What does this MR do?

Needed for !50823 (merged)

Contributes to #294266 (closed)

Add classes for parsing an alert payload example and provide the list of all possible fields and information needed to build the rest of the custom mapping.

No changelog is required. The entire functionality is hidden behind a multiple_http_integrations_custom_mapping feature flag.

Screenshots (strongly suggested)

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