Allow configuration to mount extra volume to /data

[jltobler]

I have an NFS share that contains the contents of my Minecraft server and would like to use it as the volume that gets mounted to /data directory in the container. As far as I can tell, it is not possible to configure the Minecraft Helm chart in this manner because an emptyDir volume is always mounted when the PVC is disabled and thus a conflict occurs.

To allow any defined extra volume to be mounted to the /data directory, I propose a new extraDataVolume configuration be introduced.

Thanks in advance for taking a look.

Example use with NFS:

persistence:
  extraDataVolume: nfs

extraVolumes:
    volumes:
      - name: nfs
        nfs:
          server: example.nfs.server
          path: /data/minecraft