add missing witness schema (#913)

Author: Ryun1

CIP-0108/cip-0108.common.schema.json

@@ -44,6 +44,26 @@
                 }
             }
         },
+        "Witness": {
+            "title": "Witness",
+            "description": "A witness proving that the author endorses the content of the metadata",
+            "type": "object",
+            "properties": {
+                "witnessAlgorithm": {
+                    "title": "WitnessAlgorithm",
+                    "type": "string",
+                    "enum": [ "ed25519", "CIP-0008"]
+                },
+                "publicKey": {
+                    "title": "PublicKey",
+                    "type": "string"
+                },
+                "signature": {
+                    "title": "Signature",
+                    "type": "string"
+                }
+            }
+        },
         "body": {
             "title": "Body",
             "description": "The body of the metadata document that is hashed to produce a signature (CIP-100)",