feat: allow assets to be managed externally (#18093)

* feat: allow assets to be managed externally

* Use server_default

superset/models/slice.py

@@ -25,7 +25,16 @@ import sqlalchemy as sqla
 from flask_appbuilder import Model
 from flask_appbuilder.models.decorators import renders
 from markupsafe import escape, Markup
-from sqlalchemy import Column, DateTime, ForeignKey, Integer, String, Table, Text
+from sqlalchemy import (
+    Boolean,
+    Column,
+    DateTime,
+    ForeignKey,
+    Integer,
+    String,
+    Table,
+    Text,
+)
 from sqlalchemy.engine.base import Connection
 from sqlalchemy.orm import relationship
 from sqlalchemy.orm.mapper import Mapper
@@ -83,6 +92,8 @@ class Slice(  # pylint: disable=too-many-public-methods
     last_saved_by_fk = Column(Integer, ForeignKey("ab_user.id"), nullable=True)
     certified_by = Column(Text)
     certification_details = Column(Text)
+    is_managed_externally = Column(Boolean, nullable=False, default=False)
+    external_url = Column(Text, nullable=True)
     last_saved_by = relationship(
         security_manager.user_model, foreign_keys=[last_saved_by_fk]
     )