message: Add db_engine_spec for Druid
  The `druiddb` pypi package provides a dbapi and sqlalchemy dialect for
  Druid. This PR hooks adds some superset-specific conf.
diff: @@ -1217,6 +1217,12 @@ class ImpalaEngineSpec(BaseEngineSpec):
  return schemas
  +class DruidEngineSpec(BaseEngineSpec):
  + """Engine spec for Druid.io"""
  + engine = 'druid'
  + limit_method = LimitMethod.FETCH_MANY
  +
  +
  engines = {
  o.engine: o for o in globals().values()
  if inspect.isclass(o) and issubclass(o, BaseEngineSpec)}
  