Use trait instead of class for Entity Bridges; add OptionList trait

[colemanw]

Overview

Improves APIv4 categorization of entities, which is then used to improve the Search Kit UI.

Before

BridgeEntity is a class.
SearchKit shows all entities.

After

EntityBridge and OptionList are traits.
SearchKit list of entities excludes less useful ones.

Technical Details

Traits allow each entity to have > 1 type
This excludes both EntityBridge and OptionList types from the main search dropdown.

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[eileenmcnaughton]

Rationale makes sense. Test cover is good here