[YSQL] Support Audit Logging

[m-iancu]

Support Postgres-compatible audit logging in YSQL based on pgAudit.

For instance, to enable auditing on a database run:

CREATE EXTENSION IF NOT EXISTS pgaudit;

Then the same configuration options as in regular pgAudit should work.
In YSQL the options are configurable either per cluster using the ysql_pg_conf_csv flag, or per-session using the relevant session variables.

For instance, to enable auditing of DDLs and raising the log level to notice:

• Cluster-level config

  ysql_pg_conf_csv='"pgaudit.log='DDL'",pgaudit.log_level=notice'
  

• Session-level config

  SET pgaudit.log='DDL';
  SET pgaudit.log_level="notice";
  

[m-iancu]

Fixed by fb7ce2b.