Add Conservative Rasterization option to RenderingDevice

[Lateasusual]

Describe the project you are working on

GPU Voxelizer experiment

Describe the problem or limitation you are having in your project

I was looking into how VoxelGI could be improved, to allow ShaderMaterials (and more complex Basematerial3Ds) to be included properly.
VoxelGI currently ignores these materials, since it is baked on the CPU and can't evaluate shaders.

I'm not ready to create a whole GPU voxelizer, but this is one of the things that could be helpful in the future if somebody intends to (as well as for a few other graphics algorithms, e.g. GPU occlusion culling)

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Add an option to the rasterization state setting of render pipelines to enable conservative rasterization.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Implementation PR: godotengine/godot#97933

If this enhancement will not be used often, can it be worked around with a few lines of script?

Not in a few lines - it can be worked around with some relatively complex additional calculation: e.g. this chapter from GPU Gems but most modern Desktop GPU's have hardware support for it, which greatly simplifies implementation.

Is there a reason why this should be core and not an add-on in the asset library?

Can't be an addon, since it affects the low-level RD API.
This has no impact on higher level rendering or visuals at the moment.