Document the purpose of having common.sql

[potiuk]

Body

The Common.sql package was created in order to provide a common interface for DBApiHooks to return the data that will be universally used in a number of cases:

• CommonSQL Operators and Sensors
• (future) lineage data where returned hook results can follow the returned data for column lineage information

This should be better documentedi in common.sql that this is the goal that common.sql achieves

Committer

• I acknowledge that I am a maintainer/committer of the Apache Airflow project.