gok overwrite: add gaf (gokrazy archive format) output

[damdo]

It adds support within gok overwrite for the gaf output format. gaf is the new gokrazy archive format output type. It is immagined for storage and upgrade purposes. A single, uncompressed zip archive file that contains:

• the MBR image (mbr.img)
• the boot image (boot.img)
• the root image (root.img)
• the SBOM of the gokrazy build (sbom.json)

It is lighter in size than a "full" disk image (it doesn't have the partition filling bits). It contains all the necessary bits to upgrade a gokrazy instance (all it takes is the 3 imgs). It can be directly accessed for a single file extraction (non compressed zip property). It can be easily unarchived by gokrazy appliances (cheap unzipping). It is easy to verify what the build contains (SBOM alongside to be read).

Users can now generate a gaf output by issuing

gok overwrite --gaf gokrazy-build.gaf

[damdo]

Round of feedback addressed. The commit is wip, I'll rebase this in the corresponding commits before merging, when the PR looks good.

[damdo]

Feedback included and rebase done.