Add support for PostgreSQL Audit Extension logs

[narph]

Integration release checklist

The PostgreSQL Audit Extension (or pgaudit) provides detailed session and/or object audit logging via the standard logging facility provided by PostgreSQL. More info here https://github.com/pgaudit/pgaudit

All changes

• [ ] Change follows the contributing guidelines
• [ ] Supported versions of the monitoring target are documented
• [ ] Supported operating systems are documented (if applicable)
• [ ] Integration or System tests exist
• [ ] Documentation exists, useful guidelines to follow
• [ ] Fields follow ECS and naming conventions
• [ ] At least a manual test with ES / Kibana / Agent has been performed.
• [ ] Required Kibana version set to:

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)