[Spaces] Allow setting that a specific space doesnt show the child space rooms

#### Is your suggestion related to a problem? Please describe.
I have a quite huge space tree (28 spaces and more to come) and I currently use the spaces deeply neested. I have some spaces (e.x.: An element space) where I want to show rooms of the sub spaces (aka ios, android, web). However I also have some spaces where I would like to only see the rooms of the current space excluding the sub spaces. This or example applies for the matrix space I have. I have a matrix space with different subspaces like clients, server, etc. However there are projects that just dont fit in a category. I can either for that have in all my spaces a "general" space besides these categories or imho a way to not show subspaces in that space would solve this more nicely.

For reference this is my current space setup. Now imagine it twice as big and you will have the space tree I plan to use when the feature lands in synapse stable:


#### Describe the solution you'd like.
A space specific setting to decide if that space shall also all rooms in the subspaces or not when selecting it.

#### Describe alternatives you've considered.
A boilerplate solution would be to add a "general" category to my tree but that quickly gets confusing when I have multiple "general" spaces at different levels.