Enhancement for SSL-support in CloudSQLExecuteQueryOperator

[moiseenkov]

The PR provides enhancement for SSL support in CloudSQLExecuteQueryOperator.

1. The previous implementation used to work with SSL certificates provided by users. If those files have excessive permissions, then psql raises an exception:

psql: error: connection to server at "10.40.112.2", port 5432 failed: private key file "client-key.pem" has group or world access; file must have permissions u=rw (0600) or less if owned by the current user, or permissions u=rw,g=r (0640) or less if owned by root

In order to fix this problem, the CloudSQLDatabaseHook copies those files into a /tmp/certs/ with minimal required permissions, and uses those copies instead.

2. Added integration with Google Cloud Secret Manager, so users now can store their certificates as secrets and specify a secret id in the operator.
3. Refactored system tests and removed the deprecated example DAG.