added parameters to template_fields to support upstream task ordering by xcom

[VBhojawala]

added template_fields to support upstream task ordering by xcom .

from airflow.models import DAG
from airflow.providers.postgres.operators.postgres import PostgresOperator
import os

from airflow.utils.dates import days_ago

default_args = {'start_date': days_ago(1)}

dag_name = os.path.splitext(os.path.basename(__file__))[0]

with DAG(dag_name, default_args=default_args) as dag:

    @dag.task
    def some_py_task() -> str:
        return some_str

    some_str = some_py_task()

    some_pg_task = PostgresOperator(task_id='some_pg_task', 
                     sql='select * from %s',
                     parameters=[some_str],
                     postgres_conn_id='postgres_bob')

closes : #13823

[ashb]

These parameters aren't used anywhere, so this fix isn't correct.