Expose document related event types as public API

We have constants for event stream document event types which are currently located in this internal interface: https://github.com/xwiki/xwiki-platform/blob/xwiki-platform-15.10/xwiki-platform-core/xwiki-platform-eventstream/xwiki-platform-eventstream-stores/xwiki-platform-eventstream-store-common/src/main/java/org/xwiki/eventstream/store/internal/EventType.java

This makes difficult to reuse those constant as 1. it's internal and not an API, 2. the module involve to load old core dependencies that we might not want.