Add project export API schema

spec/fixtures/api/schemas/public_api/v4/project/export_status.json

+{
+  "type": "object",
+  "allOf": [
+    { "$ref": "identity.json" },
+    {
+      "required": [
+        "export_status"
+      ],
+      "properties": {
+        "export_status": {
+          "type": "string",
+          "enum": ["none", "started", "finished"]
+        }
+      }
+    }
+  ]
+}

spec/fixtures/api/schemas/public_api/v4/project/identity.json

+{
+  "type": "object",
+  "required": [
+    "id",
+    "description",
+    "name",
+    "name_with_namespace",
+    "path",
+    "path_with_namespace",
+    "created_at"
+  ],
+  "properties": {
+    "id": { "type": "integer" },
+    "description": { "type": ["string", "null"] },
+    "name": { "type": "string" },
+    "name_with_namespace": { "type": "string" },
+    "path": { "type": "string" },
+    "path_with_namespace": { "type": "string" },
+    "created_at": { "type": "date" }
+  }
+}