String definition annotation for VAST (Video Ad Serving Template) tracker event types.

This annotation defines a constrained set of string values representing different types of video advertisement tracking events as specified in the VAST standard. These event types are used to categorize and validate tracking URLs that should be fired during various stages of video ad playback and user interactions.

The annotation ensures type safety by restricting string values to only the predefined VAST event types, preventing invalid or malformed event type strings from being used throughout the video advertising pipeline.

Event types include standard video playback events (start, quartiles, complete), user interaction events (click, skip), audio control events (mute, unmute), playback control events (pause, resume), and tracking events (impression, error).