stages/oci-archive: support for annotations

Add support for arbitrary manifest annotations: allow anything
with the exception of the `org.osbuild` and `org.opencontainer`
prefixes. The former is reserved by us, the latter by the OCI
image specification. The latter specifies a set of pre defined
keys, which are not yet supported by osbuild but will be in the
future, partly via more generic options (creation time).

stages/org.osbuild.oci-archive

@@ -16,6 +16,10 @@ as the `config` option for the "OCI Image Configuration" (see [2]),
 except those that map to the "Go type map[string]struct{}", which are
 represented as array of strings.
 
+Manifest annotations can be included via the `annotation` options. Any
+`key`, `value` pair is allowed, with the exception of the pre-defined
+`org.osbuild` and `org.opencontainer` namespaces.
+
 The final resulting tarball, aka a "orci-archive", can be imported via
 podman[3] with `podman pull oci-archive:<archive>`.
 
@@ -96,6 +100,15 @@ SCHEMA_2 = r"""
           "type": "string"
         }
       }
+    },
+    "annotations": {
+      "type": "object",
+      "additionalProperties": false,
+      "patternProperties": {
+        "^(?!org.osbuild|org.opencontainer).+": {
+           "type": "string"
+        }
+      }
     }
   }
 },
@@ -250,6 +263,10 @@ def create_oci_dir(inputs, output_dir, options):
         "layers": []
     }
 
+    annotations = options.get("annotations", {})
+    if annotations:
+        manifest["annotations"] = annotations
+
     index = {
         "schemaVersion": 2,
         "manifests": []