Physics Interpolation - Add editor configuration warnings

[lawnjelly]

• For physics bodies that are on non-interpolated branches

4.x version of #103355
Helps address #103232 (comment)

Notes

• I had not encountered this before, but it may not be obvious to users that physics bodies won't work correctly on non-interpolated branches of the scene tree.
• This is simple to add and warns of the problem, and how to fix it.
• The warning will show even in projects not using physics interpolation, but in most cases users will not have been changing physics_interpolation_mode except by accident. There's not a super easy way to check the global interpolation switch in the editor (except via the project setting) as it is hard coded OFF in the editor.
• It is also possible to add some warnings at runtime for this (perhaps if the user switches interpolation mode of a branch at runtime) but seems less likely and can be added in a follow up PR.
• Feel free to suggest changes to the wording / text formatting.

[lawnjelly]

Repushed this to remove the warnings on interpolated Spatials that were children of non-interpolated nodes, as this may now be possible with the new SceneTreeFTI, or at least I would like to establish that it can't be done before introducing a warning.

[Ivorforce]

Code wise this looks good.
As a usability feature, this sounds important. Let's get a review from editor folks too :)

[Ivorforce]

Since we don't seem to be getting an editor review right now - following along with #103355 (comment), it should be fine to merge without one.

[Calinou]

Tested locally, it works as expected. Code looks good to me.

Note that the warning displays even if physics interpolation is disabled in the Project Settings, but I think it's fair as it ensures projects are interpolation-ready should they enable it later on.

[Repiteo]

Thanks!