General entity riding feature

[BenCheung0422]

Is your feature request related to a problem? Please describe.
It seems that boat and horse may be added in the future, so this feature is important.
If we need to implement the logic and idea correctly and completely, this feature (https://minecraft.fandom.com/wiki/Riding) would still be needed, instead of just targeting implementing the feature in the simplest and easiest way. This behaviour and direction have been used for a long time with Load and the static class structures as examples. (Taken from a Discord message in the boat post.)

Describe the solution you'd like
We should implement the riding feature. If boat is implemented with Boat as in #134, it would not be a standardized logic, i.e. not implemented completely and correctly. There should be a map (a BiMap is recommended.) mapping the entity riding relationships. The map should be stored in a separated file in save. When the save system is rewritten, the data of the passenger can be instead recorded within an entity recursively instead of using a separated mapping. The position of the passenger is led by the mount and the boat is controlled by the player when being ridden. An entity can only be ridden and/or riding not more than one entity not being ridden by the entity itself.

[DevWolf31]

This is fine.

[BenCheung0422]

Also, #287 mentioned ridable entity, so this could be an important feature, and #557 mentioned minecart, which might be possible and more realizable if the whole entity moving control system is rewritten.