cloudapi: make fs_type required for filesystems and logical volumes

While technically this is a backwards incompatible API change, the
core blueprint schema didn't allow for empty or undefined fs_type [1,2],
so the general behaviour hasn't changed, we're just catching it early
now.

[1] de18902e3f/pkg/blueprint/disk_customizations.go (L624-L627)
[2] de18902e3f/pkg/blueprint/disk_customizations.go (L677-L680)

internal/cloudapi/v2/openapi.v2.yml

@@ -2683,6 +2683,8 @@ components:
         - $ref: '#/components/schemas/VolumeGroup'
     FilesystemTyped:
       type: object
+      required:
+        - fs_type
       properties:
         type:
           type: string
@@ -2767,6 +2769,8 @@ components:
             $ref: '#/components/schemas/LogicalVolume'
     LogicalVolume:
       type: object
+      required:
+        - fs_type
       properties:
         name:
           type: string