Add default physics material and physics material override for root CSGShape

[gaudecker]

Adds a default physics material, same that StaticBody3D has, and adds a new property to override the default physics material.

Fixes #36893

Small demo project: csg-physics-material-demo.zip

• Production edit: This closes Add physics material field for CSG objects godot-proposals#7568.

[YuriSizov]

We've discussed this in a PR meeting, and decided it would be good to open a proposal to discuss this further. The existing issue can be converted to one, but it'd be better to open a new proposal and supersede the issue. This may be adding too much to the CSG systems which were mainly created for blocking and prototyping.

[Calinou]

We've discussed this in a PR meeting, and decided it would be good to open a proposal to discuss this further. The existing issue can be converted to one, but it'd be better to open a new proposal and supersede the issue. This may be adding too much to the CSG systems which were mainly created for blocking and prototyping.

A proposal was opened: godotengine/godot-proposals#7568

[aaronfranke]

See also this proposal: godotengine/godot-proposals#7401