Create an audit log destination global configuration

In order to define the output destination for audit event logs, a new GlobalConfiguration class should be created to select that output destination. To make things simple to start, this should simply use a file name to output the audit log to which can default to $JENKINS_HOME/logs/audit/audit.log as done in JENKINS-54636.

Later on, we may add configuration screens to choose some logging settings, though that's a more complex topic.