Refactor SystemHooksService (Decrease CyclomaticComplexity)

What does this MR do?

This MR reduces the Cyclomatic Complexity in SystemHooksService class as listed here.
All event-data logic has been extracted into a set of smaller focused classes, which share behaviour in EventInfos::Base class and each adding own specific data, thereby avoiding big conditionals.
Specs have been just slightly updated - not to rely on a private method (build_event_name), but rather on the result of the new object.
The specs in spec/services/system_hooks_service_spec.rb should also be refactored since they basically just test event data assembly

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/31362

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer