schema: metadata at the top level

Allows a new object under the top-level key `metadata` which contains a
`generators` property. This property is a list of all generators involved
in the creation of a manifest.

Each generator can add its name and version to this list.

Signed-off-by: Simon de Vlieger <supakeen@redhat.com>

test/data/manifests/fedora-boot.json

@@ -1,5 +1,13 @@
 {
   "version": "2",
+  "metadata": {
+    "generators": [
+      {
+        "name": "osbuild-mpp",
+        "version": "unknown"
+      }
+    ]
+  },
   "pipelines": [
     {
       "runner": "org.osbuild.fedora39",

test/data/manifests/fedora-boot.mpp.yaml

@@ -2,6 +2,10 @@
 # Noticed the org.osbuild.test that was added.
 
 version: '2'
+metadata:
+  generators:
+    - name: osbuild-mpp
+      version: unknown
 pipelines:
   - mpp-import-pipelines:
       path: fedora-vars.ipp.yaml