introduce getSpaceHierarchy cache

[WilsonLe]

Describe your problem

With apps that call Api.getSpaceHierarchy often, it would be beneficial to have a built in cache of room hierarchy.

Describe your ideal solution

Introduce room hierarchy box in database cache

[krille-chan]

Actually I'm not sure. The hierarchy can also be cached by the app. When you use this endpoint the consumer would just expect to get the newest version. Such complexity with caching could be added to dozens of other endpoints as well which would blow up the sdk too much. But that's only my opinion. What do the others think? @famedly/instant-messaging