Support for external PostgreSQL cluster client-side High Availability

[AymericP35]

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Feature Summary

Add ability to deploy an AWX instance with an external PostgreSQL cluster using client-side High Availability mode.

According to the PostgreSQL documentation, when using client-side High Availability, you must change the target_session_attrs default value any to read-write in order to select the primary postgreSQL server.

To allow such behavior, I suggest the following :

• Adjusting the Jinja template credentials.py.j2 to allow target_session_attrs option ;
• Add awx_postgres_target_session_attrs var in database_configuration.yml ;
• Update database-configuration.md documentation.

[moonrail]

fyi, a non-configurable default approach was proposed in 2022 and actually approved, but never merged: #928

I've just now retested if AWX 24 can run with readonly PostgreSQL-Instances and the answer is: No, it cannot.
There is no benefit to not setting target_session_attrs to read-write. And there is no harm in doing so.