stages(sgdisk): support label option

We only support `gpt` here so it would seem this option doesn't
make much sense to add, but it will make it so that the mpp-define-images
from osbuild-mpp can be passed in to `org.osbuild.sgdisk` just as it
can be passed in today to `org.osbuild.sfdisk`.

stages/org.osbuild.sgdisk

@@ -35,6 +35,11 @@ SCHEMA_2 = r"""
       "type": "boolean",
       "default": true
     },
+    "label": {
+      "description": "The type of the partition table. Only 'gpt' supported.",
+      "type": "string",
+      "enum": ["gpt"]
+    },
     "partitions": {
       "description": "Partition layout ",
       "type": "array",

test/data/manifests/fedora-ostree-image.json

@@ -1216,7 +1216,7 @@
           }
         },
         {
-          "type": "org.osbuild.sfdisk",
+          "type": "org.osbuild.sgdisk",
           "devices": {
             "device": {
               "type": "org.osbuild.loopback",

test/data/manifests/fedora-ostree-image.mpp.yaml

@@ -143,7 +143,7 @@ pipelines:
           filename: disk.img
           size:
             mpp-format-string: '{image.size}'
-      - type: org.osbuild.sfdisk
+      - type: org.osbuild.sgdisk
         devices:
           device:
             type: org.osbuild.loopback