Support for different floppy image formats

[dbalsom]

Currently it looks like we must supply a raw sector image to be read to mount a FAT12 filesystem.

Over the history of PC floppy disks, there were a lot of unusual disks mastered, often for copy-protection purposes. Varying sector counts and sizes per track, overlapping sectors, etc. Raw sector images can only hold information about a standard floppy with 512 bytes per sector, and the same number of sectors per track.

I am not proposing that you support the dozens of different floppy image formats out there, but I was thinking about something like a SectorProvider trait that a user could implement for alternate disk formats, as a way for rust-fatfs to query disk layout and request sector data in a standard way.