Deregister entity tree API in entity store

[albinsuresh]

Is your feature request related to a problem? Please describe.

The existing tedge_api::entity_store::EntityStore impl doesn't have any APIs to remove an entity, along with its children from the entity store. Implementing is also not trivial as the entities are currently stored in a HashMap and retrieving all the children of a given entity would involve multiple iterations over the entire map looking for each @parent, which isn't efficient.

Describe the solution you'd like

Add a function with the following signature that accepts the topic id of the parent entity to be removed, along with its children, and returns the list of all the entities that were removed:

deregister(topic_id: &EntityTopicId) -> Result<Vec<EntityTopicId>, Error>

To make the retrieval of the child tree more efficient, a tree of EntityTopicIds can also be maintained along with the HashMap, which can further be used in the deregsiter API implementation. The same tree structure can be used to support search APIs to be added in future, to fetch things like all immediate child devices of a given device, all services of a given device etc.