[feature] Ability to add additional Volumes

[SimulatedGREG]

The ability to add additional Volumes to the backstage pod would allow for several things, but my immediate request would be for writing a GitHub App credentials file from a Secret.

[vinzscam]

I think you should be able to do this using https://github.com/vinzscam/backstage-chart#pass-extra-configuration-files
If you create a new configmap containing:

integrations:
  github:
    - host: github.com
      apps:
        - appId: app id
           allowedInstallationOwners: ['GlobexCorp']
           clientId: client id
           clientSecret: client secret
           ...

or creating a configmap that looks like

integrations:
  github:
    - host: github.com
      apps:
        - appId: ${GITHUB_APP_ID}
           allowedInstallationOwners: ['GlobexCorp']
           clientId: ${GITHUB_CLIENT_ID}
           clientSecret: ${GITHUB_CLIENT_SECRET}
           ...

and then store the environment variables as secret as described here