Serialize ExternalTaskMarker dag dependency

### Body Related to: https://github.com/apache/airflow/issues/42367 Final Goal: `SerializedDagModel.get_dag_dependencies()` is not returning linked objects related to `ExternalTaskMarker`. This is useful for the front-end to be able to display that a specific dag /task depends on an external task. Consideration: - This impacts how we serialize dags, especially the `dag_dependencies` fields. ### Committer - [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.