Evidence collection - add serializers, entities and JSON schemas

What does this MR do?

This is an effort to split a MR !17217 (merged) that's going to be bigger into smaller deliverable chunks.

This small MR adds a serializer and a few entity classes, as well as their rspecs. They will be used to form the JSON object that will be stored into the future evidences table, which will be the focus of the MR this is extracted from.

This MR also adds some JSON schema files that will be used to check the JSON format of the future Evidence object

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides

Relates to #26019 (closed)