Implement the Entity Channel and the New Spatial Channel

[indiest]

Corresponding to channeld's Entity Channel feature.

Things to implement in ChanneldUE:

• Use the new data structure for the spatial channels;
• Codegen for the entity channel data;
• New spawn/destory logic;
• New handover logic;
• Interest management for the entities;
• Well-Known Object (a.k.a. AlwaysRelevant)
• EntityAOI