Add path parameter to get_quota interface

[Vigilans]

This PR adds path parameter to DavFileSystem trait's get_quota interface.

There are many cases when we use a folder like /media to export as webdav file root, but subfolders in /media are mounted with other filesystems:

/media -> rootfs
/media/dirA -> external ext4 system
/media/dirB -> external NTFS system

Currently, get_quota only collects quota information of root folder (the /media), even the client is querying quota information of path /media/dirA or /media/dirB.

When using clients like RaiDrive in Windows, it will report disk usage just like normal physical disks, and we expect it to show the correct disk usage of drive /media/dirA or /media/dirB (maybe some TBs large), but not the /media from rootfs `(maybe just tens of GBs).

This PR adds a path parameter to get_quota interface, so its implementations could make use of the path information to return the correct quota information of corresponding disk.

This PR depends on #11 to return quota information for all directory path, instead of return only for root (/) path, and is implemented by miquels/webdav-server-rs#19.