support local disk backend

[adamqqqplay]

Currently nydus supports 4 different backends, they are registry, localfs, oss and s3, which can already meet the existing needs. But in Confidential Computing or Hybrid Image scenarios, containers may need to read data from block devices, so we could add local disk backends to nydus to support more scenarios.

The following components need to be supported:

• [ ] nydusify
• [ ] acceld
• [x] nydusd
• [ ] nydus-snapshotter