Proposal: Rename storage.disk to storage.fs

[Xuanwo]

We are using storage.disk to refer to the storage backend provided by local fs.

However, it's incorrect and confusing.

• disk often refers to block storage like HDD or SSD. But databend can't work on disk directly. We need a file system over those block devices.
• The fs may be NFS or FUSE powered by ceph or s3fs, but disk can't reflect that possibility.

So I propose to rename all disk in our current codebase and docs to fs instead.