idea: "serve" /.datalad-fuse-wtf file

[yarikoptic]

Inspired by uncertainty either currently fused mount point is running the expected version of datalad-fuse in #51 (comment)

We could provide read-only open (and whatever else is needed for cat to work) for /.datalad-fuse-wtf file which would just provide output of datalad.api.wtf which should also include version of the datalad-fuse extension etc, which would be useful for troubleshooting etc.

edit: we might want also some section on options of fuse mount point, which mode it is etc. May be should be actually "extended" in datalad wtf to allow for extensions to provide any relevant to them information within datalad wtf output