[FEATURES] Make Juicefs runtime can use memory-backed empty dir to prepare cache tiered

[cheyang]

What feature you'd like to add:

By default, /dev/shm only uses 1/2 memory, but we can set it as demand by the following way. Jindofsx has supported it, please refer it. Thanks.

apiVersion: v1
kind: Pod
metadata:
  name: buildah-emptydir 
spec:
  containers:
    - name: buildah
      image: quay.io/buildah/stable:v1.23.1
      command: ["sleep", "infinity"]  
      volumeMounts:
      - mountPath: /var/lib/containers
        name: container-storage
  volumes:
  - name: container-storage
    emptyDir:
     medium: Memory
     sizeLimit: 1Gi

Why is this feature needed: