Disk pressure on large file uploads

[alexanderccc]

During uploads of large files (depends on available host storage, in my case it's 30GB), the node receives a DiskPressure taint.
This is due to limited space available on the node itself, and Kong utilizing an emptyDir volume for it's tmp dir.
Adding a limit causes the pod to be evicted.

This should be easily resolved by attaching a volume to the tmp directory, and directing the tmp files to a different storage device, but the chart doesn't have support for this.

Something like the below could allow for some increased flexibility in handling this

{{- define "kong.volumes" -}}
- name: {{ template "kong.fullname" . }}-tmp
  {{- if .Values.deployment.tmpDir.customVolume }}
  {{- .Values.deployment.tmpDir.customVolume | toYaml | nindent 2 }}
  {{- else }}
  emptyDir:
    sizeLimit: {{ .Values.deployment.tmpDir.sizeLimit }}
  {{- end }}
...
{{- end }}

I'm thinking this could be useful in other situations as well, like setting the emptyDir to the memory medium and utilizing the node memory for quicker file uploads.

If this would be acceptable I can prep this and open a PR