Add support for distro root filesystem other than ext4

Nobody has claimed this yet.

I know ext4 is the well-supported standard, but other filesystems have so many additional features and better performance in several use cases.

From what I understand from the logs, ext4 filesystem type is somewhat hardcoded in WSL, here is what happens when I try to run a distro registered in-place with a btrfs filesystem on the VHDX:
```
[ 61.888919] EXT4-fs (sdc): VFS: Can't find ext4 filesystem
[ 61.889230] WSL (1) ERROR: Mount:1771: mount(/dev/sdc, /distro, ext4, 0x0, discard,errors=remount-ro,data=ordered
[ 61.889233] ) failed 22
[ 61.889856] WSL (1) ERROR: Invalid argument @main.cpp:2881 (ProcessMessage)
```

**Describe the solution you'd like**
The ideal world is something like WSL detects (or tells the kernel to detect) the filesystem present in the distro vhd.
Another solution could be to specify it but I don't see where you could specify such a per-distro setting outside the vhd itself (maybe in the registry?)

**Describe alternatives you've considered**
As btrfs is already supported in the original WSL2 kernel, I can mount an external btrfs vhd with `wsl --mount`, then chroot or something like that, but it's very inconvenient to do every time.

**Additional context**
I don't think this is the correct place to list the benefits of using other filesystems as distro's root, but my primary objective now is to use btrfs for its better performance, ability to enable compression and to take (and send/receive) snapshots.

Start with the distro mount path identified near main.cpp:2881 and reproduce the failure using a distro VHDX formatted with btrfs. Compare that path with the existing wsl --mount behavior; done means an in-place distro root filesystem can be mounted without assuming ext4.

Written by the indexing model from the issue text.