feat: add migration metadata to common schema

This adds a new field, `migrationMetadata`, which has two sub-fields:

- `originalDocument`: the original document, in case we ever need it
- `hypercore`: metadata about the document in the old Hypercore

proto/common/v1.proto

@@ -13,6 +13,18 @@ message Common_1 {
   google.protobuf.Timestamp updatedAt = 4 [(required) = true];
   VersionId_1 originalVersionId = 5;
   bool deleted = 6 [(required) = true];
+
+  message MigrationMetadata {
+    bytes originalDocument = 1;
+    message HypercoreMetadata {
+      optional string rootHashChecksum = 1;
+      optional string signature = 2;
+      optional string coreKey = 3;
+      optional uint32 blockIndex = 4;
+    }
+    optional HypercoreMetadata hypercore = 2;
+  }
+  optional MigrationMetadata migrationMetadata = 7;
 }
 /* ignored fields and differences from common.json jsonSchema
  * id is a byte buffer here and a string in jsonSchema

schema/common/v1.json

@@ -46,6 +46,40 @@
     "deleted": {
       "description": "Indicates whether the document has been deleted",
       "type": "boolean"
+    },
+    "migrationMetadata": {
+      "description": "Metadata about the migration of this document from Mapeo Legacy",
+      "type": "object",
+      "properties": {
+        "originalDocument": {
+          "description": "Hex-encoded raw document from Mapeo Legacy",
+          "type": "string"
+        },
+        "hypercore": {
+          "type": "object",
+          "properties": {
+            "rootHashChecksum": {
+              "description": "Checksum of the root hash of the document",
+              "type": "string",
+              "minLength": 1
+            },
+            "signature": {
+              "description": "Signature of the document",
+              "type": "string",
+              "minLength": 1
+            },
+            "coreKey": {
+              "description": "Core key of the document",
+              "type": "string",
+              "minLength": 1
+            },
+            "blockIndex": {
+              "description": "Block index of the document",
+              "type": "number"
+            }
+          }
+        }
+      }
     }
   },
   "required": [