Adding v0 import/export fields to dashboard metadata schema (#13425)

Co-authored-by: michelle_thomas <michelle.thomas@airbnb.com>
diff --git a/superset/dashboards/schemas.py b/superset/dashboards/schemas.py
index 360f94f..53f18c0 100644
--- a/superset/dashboards/schemas.py
+++ b/superset/dashboards/schemas.py
@@ -119,6 +119,9 @@
     stagger_time = fields.Integer()
     color_scheme = fields.Str(allow_none=True)
     label_colors = fields.Dict()
+    # used for v0 import/export
+    import_time = fields.Integer()
+    remote_id = fields.Integer()
 
 
 class BaseDashboardSchema(Schema):