cephfs: go io/fs interface

[iggy]

It would be nice if the cephfs package could support the fs.FS interface. It would make it easier to do operations on the filesystem contents without mounting it (namely walkdir https://pkg.go.dev/io/fs#WalkDir )

https://benjamincongdon.me/blog/2021/01/21/A-Tour-of-Go-116s-iofs-package/

[phlogistonjohn]

Note to self: This was briefly discussed in #327 prior to the feature being merged in Go.