Apply specific ID collation to root_dag_id too

In certain databases there is a need to set the collation for ID fields
like dag_id or task_id to something different than the database default.
This is because in MySQL with utf8mb4 the index size becomes too big for
the MySQL limits. In past pull requests this was handled
[apache#7570](apache#7570),
[apache#17729](apache#17729), but the
root_dag_id field on the dag model was missed. Since this field is used
to join with the dag_id in various other models ([and
self-referentially](https://github.com/apache/airflow/blob/451c7cbc42a83a180c4362693508ed33dd1d1dab/airflow/models/dag.py#L2766)),
it also needs to have the same collation as other ID fields.

This can be seen by running `airflow db reset` before and after applying
this change while also specifying `sql_engine_collation_for_ids` in the
configuration.

Other related PRs
[apache#19408](apache#19408)

airflow/migrations/versions/0045_b3b105409875_add_root_dag_id_to_dag.py

@@ -27,6 +27,8 @@
 import sqlalchemy as sa
 from alembic import op
 
+from airflow.migrations.db_types import StringID
+
 # revision identifiers, used by Alembic.
 revision = 'b3b105409875'
 down_revision = 'd38e04c12aa2'
@@ -37,7 +39,7 @@
 
 def upgrade():
     """Apply Add ``root_dag_id`` to ``DAG``"""
-    op.add_column('dag', sa.Column('root_dag_id', sa.String(length=250), nullable=True))
+    op.add_column('dag', sa.Column('root_dag_id', StringID(), nullable=True))
     op.create_index('idx_root_dag_id', 'dag', ['root_dag_id'], unique=False)