stages/oci-archive: support additional layers

In addition to the required base layer, provided via the the input
of the same name, the oci-archive stage now accepts up to nine
additional layers that get added on top of each other, sorted in
ascending order, i.e. `layer.1` to `layer.9`.

Adapt the `fedora-ostree-container` example manifest so that the
ostree commit is now in a separate layer, which makes it possible
to share the base layer between different commits container.

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

@@ -893,7 +893,13 @@
           "options": {
             "language": "en_US"
           }
-        },
+        }
+      ]
+    },
+    {
+      "name": "container-ostree",
+      "build": "name:build",
+      "stages": [
         {
           "type": "org.osbuild.ostree.init",
           "options": {
@@ -932,6 +938,13 @@
               "references": [
                 "name:container-tree"
               ]
+            },
+            "layer.1": {
+              "type": "org.osbuild.tree",
+              "origin": "org.osbuild.pipeline",
+              "references": [
+                "name:container-ostree"
+              ]
             }
           },
           "options": {