Allow plugins to add search types to event execution

[kroepke]

Description

To efficiently make decisions, some EventModifier implementations need to calculate additional aggregations. This change allows adding search types to the main event query. Currently this only makes sense for aggregation event processors, so that's the only place we added the capability for now.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Refactoring (non-breaking change)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.