feat: Observer driven seeding + forking

[Bluefinger]

Adding observer driven seeding & forking to targeted child entities, with the idea to be able to streamline the process of managing more complex relationships between source entities and target children entities. This way, many different sources can propagate deterministically new seeds, and the process can be controlled manually by emitting events or automatically when a Source entity is reseeded.

TODO

• Add more observer events for more granular operations
• See if generics can be reduced or removed entirely for events
• Gate the observer code behind a feature flag, as this is likely to be reworked/refactored with API breaking changes.
• Documentation and tutorial for the observer driven flow.