[3.x] Physics Interpolation - Add editor warning for non-interpolated PhysicsBody

[lawnjelly]

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.
• Also relevant to 4.x.

[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.

[lawnjelly]

Linked @Ivorforce as reviewed #103504 and this is the same for 3.x.

[Ivorforce]

It's fine to me, but i was hoping to get an editor review on #103504 so we could transitively accept this one too

[lawnjelly]

There's no need imo, changing the configuration warnings is pretty straight forward (it's more the wording etc).

It's also a catch22, you'll likely not get an editor review as editor guys aren't familiar enough to approve physics interpolation. 😁

Also bear in mind the relevant teams will get flagged automatically, if it doesn't get a prompt review things just tend to languish in approval purgatory without pinging a specific member.

[lawnjelly]

Thanks!

[lawnjelly]

Cherry-picked for 3.6.1.