Add extraArgs for mdadm

[fidgetingbits]

This PR adds a simple extraArgs option for mdadm that allows you to pass something like --assume-clean during disk creation. A simple test that passes an argument is also included.

Being able to pass --assume-clean, for the sake of example, allows you to do things like more easily avoid initial resync during raid array creation, which can be multi-hour process depending on the size of the array, and otherwise a bit non-intuitive to cancel using /proc/mdstat.

[Enzime]

Can you run nix fmt . on each commit?

[fidgetingbits]

Done

[Enzime]

LGTM

Thanks for the contribution :)