Support sfdisk backend for partitioning

[jlebon]

sfdisk is part of util-linux and has supported GPT partitioning for a while now. It'd be desirable to support using sfdisk instead of sgdisk for partitioning for OSes that want to shed that dependency.

Proposal:

1. add a new sfdisk package that implements the same API as the sgdisk one
2. support a compile option to change the default backend to use from sgdisk to sfdisk
3. support a kernel argument option (e.g. ignition.partition-backend=(sfdisk|sgdisk)) to override compile-time defaults
4. once we have some confidence in the code, change the upstream default to sfdisk and declare the sgdisk backend deprecated
5. after a reasonable window, drop support for the sgdisk backend

On the FCOS side, we can start using the sfdisk backend in some of the non-production streams to get some early testing, and if deemed necessary, bake it in the next stream as well.

[jlebon]

cc @tormath1 @pothos for thoughts on this from the Flatcar Linux side.

[pothos]

With sfdisk we can skip running partx here: https://github.com/coreos/ignition/blob/3b3a27b64541d01b7bb18f5f923ca3a13dcea98e/internal/exec/stages/disks/partitions.go#L545 (And later delete it when only sfdisk woud be supported)

[travier]

We'll have to update things downstream as well. Filed: https://github.com/coreos/fedora-coreos-tracker/issues/1787

[travier]

Found some examples on how to pass commands to sfdisks:

• https://man7.org/linux/man-pages/man8/sfdisk.8.html#EXAMPLES
• https://stackoverflow.com/questions/12150116/how-to-script-sfdisk-or-parted-for-multiple-partitions