Add custom navmesh baking option to NavigationRegion3D

[TypeOverride]

Describe the project you are working on

Environment Navigation Mesh

Describe the problem or limitation you are having in your project

The Navmash baking has no option for custom navmeshes to generate. No mode like "Custom" or "Solid" at Sampling on "NavigationRegion3D".

Navigation-Links are not what I am looking for. I want to create in Blender custom Navmeshes as meshes (each for every environment part)
and then bake them into one, inside Godot.

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

It is simple to use, and fits well with "source geometry mode" like looking for "navigation_mesh_source_group"-tags and baking them in the way needed.

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

I have implemented this before for Godot 3:

godotengine/godot#30190

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

Yes, no. Not tested it, but for sure you can create a merged Navmesh in some way, and add it to the "NavigationRegion3D" via code.
Also there is the Blender suffix -navmesh. But it creates a NavigationRegion3D node. Also you can't export a Navmesh in Godot when going over .blend file import settings. Maybe also a thing.

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

The reason is because it would be super simple to setup all in a node structure. Give it the group name "navigation_mesh_source_group", and even objects with a mesh as navmesh could be baked into one big static navmesh. So it can all be seperated blend files.

[CarpenterBlue]

I think you can use the post-import script to extract the navmesh.