feat: Override superset secret key

[michaeljguarino]

Summary

Looks like this isn't set properly via the chart, setting ourselves manually here.

Test Plan

local link

Checklist

• [ ] No images hosted from dockerhub
• [ ] Are dashboards present to understand the health of the application. There must be at least 1 of these
  • [ ] all databases should have dashboards
  • [ ] ideally also have at least cpu/mem utilization dashboards for webserver tier of the app
  • [ ] you can use plural from-grafana to convert a grafana dashboard found via google to our CRD
• [ ] Are scaling runbooks present
  • [ ] all databases must have scaling runbooks
  • [ ] you can use the charts in pluralsh/module-library to accelerate this
• [ ] do you need to add config overlays?
  • [ ] inputing secrets
  • [ ] configuring autoscaling
• [ ] If there’s a web-facing component to the app, we need to support OIDC authentication and setting up private networks if no authentication option is viable
• [ ] All major clouds must be supported
  • [ ] Azure
  • [ ] AWS
  • [ ] GCP