Request for "ipfs filestore ls" that list filenames (not objects)

Currently the only way to list which files are in the filestore (that I have found) is by running `ipfs filestore verify`

**What is the usecase**
- Which files do I have added in the store (that I shouldn't touch)
- What is the hash of the full file

**What should be returned**
- Basic implementation would be to only list filenames that have been placed in the filestore.
- Extended implementation would be to also return IPFS hash for the full file - this would be calculated from existing block hashes, without recalculating the whole file. (this should also make it possible to get the file hash without actually having the file on disk)