Support filesystems

[kerthcet]

What would you like to be added:

Support filesystems with the uri protocol as pvc://, this is compatible with distributed cache systems like fluid in the future.

Why is this needed:

Completion requirements:

This enhancement requires the following artifacts:

• [x] Design doc
• [x] API change
• [x] Docs update

The artifacts should be linked in subsequent comments.

[kerthcet]

/kind feature