WIP: Extract prometheus specific parsing from `Alerting::Alert`

What does this MR do?

That is a part of implementing Generic Alert Endpoint.

At the moment Alerting::Alert class is responsible for both, an alert presentation, and parsing a payload.

Extracting the parser into a separate class will allow extending its functionality to support different alert payloads.

The current parser works only for Prometheus payload. That's why it returns :prometheus as a service. Next step will be splitting the parser into a Prometheus specific payload and a generic one.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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