### What changes were proposed in this pull request?
This PR (SPARK-31394) aims to add a new feature that enables mounting of Kubernetes NFS volumes. Most of the codes are just slight modifications from the existing codes for EmptyDir/HostDir/PVC support.
### Why are the changes needed?
Kubernetes supports various kinds of volumes, but Spark for Kubernetes supports only EmptyDir/HostDir/PVC. By adding support for NFS, we can use Spark for Kubernetes with NFS storage.
In order to use NFS with the current Spark using PVC, the user needs to first create an empty new PVC with NFS. Kubernetes' NFS provisioner will create a new empty dir in NFS under some pre-configured dir for this PVC, for example, `/nfs/k8s/sjcho-my-notebook-pvc-dce84888-7a9d-11e6-b1ee-5254001e0c1b`. Then the user should add files to process in the newly created PVC using some file-copying job, and then run the desired Spark job using that populated PVC. And then to get the final results out, the user should run another file-copying job.
This in theory works, but for data analysis tasks, is quite cumbersome. With this change, one could simply use existing files in NFS, say `/nfs/home/sjcho/myfiles10.sstable` from the Spark job directly, and also write the results directly to some existing dir under NFS such as `/nfs/home/sjcho/output`.
This PR doesn't use any features other than the features already provided by Kubernetes itself, so there should be no compatibility issues (other than limited by k8s) between the wide variety of NFS choices. This PR merely enables an existing volume type `nfs` supported officially by Kubernetes, just like Spark is currently supporting `hostPath` and `persistentVolumeClaim` right now.
### Does this PR introduce any user-facing change?
Users can now mount NFS volumes by running commands like:
```
spark-submit \
--conf spark.kubernetes.driver.volumes.nfs.myshare.mount.path=/myshare \
--conf spark.kubernetes.driver.volumes.nfs.myshare.mount.readOnly=false \
--conf spark.kubernetes.driver.volumes.nfs.myshare.options.server=nfs.example.com \
--conf spark.kubernetes.driver.volumes.nfs.myshare.options.path=/storage/myshare \
...
```
### How was this patch tested?
Test cases were added just like the existing EmptyDir support.
The code were tested using minikube using the following script:
https://gist.github.com/w4-sjcho/4ba48f8c35a9685f5307fbd46b2c0656#file-run-test-sh
The script creates a new minikube cluster, launches an NFS server inside the cluster, copy `README.md` file to the NFS share, and run `JavaWordCount` example against the file located in NFS.
Closes#27364 from w4-sjcho/master.
Authored-by: Seongjin Cho <sjcho@wisefour.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
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