Make SubViewportContainer's adjustments optional

[KoBeWi]

This PR improves documentation about properties modified by SubViewportContainer and makes this behavior optional by adding adjust_viewport_properties property.
Closes #56502
Closes #23729

[kubecz3k]

I would argue it probably would be better to simply the behavior by ensuring Godot is not changing the value of update_mode every time the visibility is changing.
It should either:

• simply disable the processing using internal flag which is not exposed to gdscript
• or don't do anything since this can be achieved very easily manually in gdscript (and that's how I would expect it to work before this pr without looking at the implementation) (edit: after reconsideration I think it's not so obvious as it seemed to me at first)

[KoBeWi]

simply disable the processing using internal flag which is not exposed to gdscript

Update mode is handled on RenderingServer side. It can't be an internal flag.

EDIT:
Ah, I think I misunderstood. It could be an internal flag. The problem is that Viewport and SubViewport would need additional property that would only be modified from SubViewportContainer (and it would basically duplicate existing properties). It would add complexity for little gain. The changes in my PR are simpler and basically solve the same problem.

[kubecz3k]

Well from user perspective internal flag makes it behave exactly like they would expect (disabling processing when SubViewportContainer is hidden), while respecting update_mode (in current behavior exposing update_mode has no sense since no matter what programmer will set there, engine will always overwrite it). This way we would not need to introduce new a bit cryptic adjust_viewport_properties flag and as a result the api would be simpler/cleaner

[KoBeWi]

Rebased. I also tweaked _validate_property() to disable the properties handled by the container.

EDIT:
I also made the adjustments take effect right after SubViewport is added to the container.