Better handling for decision logs with vector

[Jimvin]

Currently OPA decision logs are treated in the same way as all other logs, by taking the values returned by OPA and adding it to the message field. The decision log is already a JSON document and flattening it into a string makes it harder to use. We should provide a way to log the decision logs as JSON so that it can be indexed better by OpenSearch. This would make building dashboards based of the authorisation events easier to build and more performant.