Use deployment slots for ingestion service function app function

[somesylvie]

DevEx/OpEx

In #1220, we implemented deployment slots for the TI service and the SFTP ingestion web app, but not for the function app. Since the function app only runs based on a cron expression and does not take external traffic, having a small downtime while it deploys doesn't actually impact the overall service uptime.

Tasks

• [ ] Implement deployment slots for the ingestion service function app
  • [ ] Make sure the production and pre-live slots are both operational (so we can roll back by swapping slots), but also make sure the pre-live slot doesn't enqueue any messages (possibly via a cron that will never run, or some kind of check, or a different env var) - we don't want to have pre-live and production both triggering SFTP calls

Additional Context

Add any other context or screenshots about the work here.

[JohnNKing]

Low risk for CA, as this would automatically recover after the SFTP ingest service is back online after a deployment (which is very brief anyhow)