OpenAPI: Add a default option for Blender's location in the setup assist

The setup assistant configuration can now contain `source: default` to
indicate that the default `blender` command should be used (which in
turn tells the Worker to find whatever Blender is available on the
system).

Choosing this option will make the setup assistant skip the path check,
and just trust that the Workers will find Blender.

Reviewed-on: https://projects.blender.org/studio/flamenco/pulls/104306
Reviewed-by: Sybren A. Stüvel <sybren@blender.org>

pkg/api/flamenco-openapi.yaml

@@ -1539,6 +1539,8 @@ components:
         - path_envvar
         # The input path was used as-is, as it points to an existing binary.
         - input_path
+        # The input path was not provided, use whatever blender is available, without checking.
+        - default
 
     WorkerRegistration:
       type: object