Make conn_id optional in SQL files when using aql.render()

[tatiana]

Context

At the moment, conn_id needs to be specified in all SQL files used by aql.render(); otherwise, Astro (0.5.1) raises the following exception:

airflow.exceptions.AirflowNotFoundException: The conn_id `None` isn't defined

The current Astro documentation is misleading since it does not include this, e.g.:
https://github.com/astro-projects/astro#setting-up-sql-files

# join_customers_and_orders.sql
SELECT c.customer_id, c.source, c.region, c.member_since,
        CASE WHEN purchase_count IS NULL THEN 0 ELSE 1 END AS recent_purchase
        FROM orders c LEFT OUTER JOIN customers p ON c.customer_id = p.customer_id

Desired behavior

Users should specify a default conn_id to apply to all files in aql.render(), and overwrite in individual files if needed. This would be consistent with specifying the conn_id` in the other operators.

Acceptance criteria

• Have a way of specifying a default conn_id when using aql.render.
• SQL files used by aql.render do not need to declare conn_id
• All the documentation and examples continue working as expected.
• There is documentation covering this feature.
• Ask @mag3141592 (bug reporter) to review the PR