Add support for parsing /proc/mdstat

The `/proc/mdstat` file contains the current status of linux [`md`](https://linux.die.net/man/4/md) devices, that are software RAID implementations provided by the kernel.

The `/proc/mdstat` file is produced by [this kernel code](https://github.com/torvalds/linux/blob/master/drivers/md/md.c#L8217), it has a plain-text format. Some examples are available in [this wiki page](https://raid.wiki.kernel.org/index.php/Mdstat).

Due to the nature of the format, it can be parsed with regular expressions. An example parser (in Ruby) is [available here](https://github.com/chef/ohai/blob/main/lib/ohai/plugins/linux/mdadm.rb).

Ideally, the output of the parsing should be a Rust data structure that contains the information available as plain text.

Thanks