Addition of granular bcachefs formatting and multi-disk support

[ProjectInitiative]

These changes provide advanced usage of bcachefs formatting options. Because bcachefs provides
a one-stop-shop for wiping, partitioning, formatting and mounting with unique features for
each step, it has been broken out into it's own type if users wish to go beyond a single
disk setup.

examples/bcachefs.nix has been updated to showcase this advanced usage and some of the
available options.

Because all three processes take place via the type object, requiring "disks" attribute set
is not longer a hard requirement if a bcachefs type is provided. A check has been added to
account for that use case.

I will note that I have not gone through the official testing process called out in the contributing guidelines quite yet. I will looking into this soon, just wanted to get this PR out there for visibility :)

I have tested provisioning a VM with a 3 disk pool with this branch using nixos-anywhere, and also running the disko tool manually on another, already provisioned VM.

[ProjectInitiative]

I am planning on re-working this to follow the mdadm and zfs arch with mult-drive support, it is proving to be a good challenge.

[Mic92]

Looking forward to it.

[ProjectInitiative]

@Lassulus You mentioned wanting to take a look at the current state. This is very rough, but this is what I currently have.

I added in some markers in the individual _create scripts (BCACHEFS POSITION and BCACHEFS_MEMBER POSITION) within bcachefs.nix and bcachefs_member.nix to track the order in which they get collated. When I load it into repl and look at the top level _create function, I see them out of order (member should come first). I am running into an order of ops issue (I think) where I am not properly tracking that disks tagged with bcachefs_member need to be processed first.

 nixosConfigurations.testmachine.config.disko.devices._create
"set -efux\n\ndisko_devices_dir=$(mktemp -d)\ntrap 'rm -rf \"$disko_devices_dir\"' EXIT\nmkdir -p \"$disko_devices_dir\"\n\n( # bcachefs pool1   /mnt/pool #\n  declare -a formatOptions=('--compression=zstd')\n  declare -a mountOptions=(verbose degraded)\n  mountpoint=/mnt/pool\n  name=pool1\n  type=bcachefs\n  echo \"Creating bcachefs device: $device\" >&2\n  \n  echo BCACHEFS POSITION\n
...
...
...

Curious your thoughts. Thanks

[ProjectInitiative]

Linking the following for visibility:
koverstreet/bcachefs-tools#308
systemd/systemd#8234

[ProjectInitiative]

Some additional pitfalls discovered while testing with real hardware:

• Using nixos-anywhere fails because when installing on systems with kernels < 6.13, UUID installations fail. A "nofail" mount would need to be added to all installs which seems hacky and not suitable for people running older kernels.
• Encryption won't work out of the box as I have not found (yet) a good way to implement the interactive passphrase required at setup even though mounting options allow for key-files and passphrase files. Not sure why initial provisioning does not support this.

Successful deployments are possible, but require additional "helpers" external to this project. I am not sure this is a feasible implementation at this time. Might be good as an experimental feature with a write up on what is needed to supplement within a user's nixos config.