Add endpoint to download combined client audit CSV

When a form definition requests a [client audit](https://docs.getodk.org/form-audit-log/), Central currently saves all audit events to the db. When a zipped CSV export [is requested](https://odkcentral.docs.apiary.io/#reference/submissions/submissions/exporting-form-submissions-to-csv), a combined audit CSV is included in the zip.

It would be helpful to have a separate endpoint to request just the combined audit CSV.

For the route, I'm thinking: projects/projectId/forms/xmlFormId/client-audit.csv` which leaves room for eventually introducing an OData endpoint.

Something to check is what happens to client audits with submissions are edited and whether they should also be available as a submission version subresource. I think they're tied to the logical submission but I'm not sure.