This adds support for Gokrazy GAF (Gokrazy Archive Format) zip
auto-updates, starting to wire up Tailscale's clientupdate mechanism
to Gokrazy's update mechanism.
Currently there's just a CLI command to update from a GAF URL,
with an --unsigned flag for use in a new natlab vmtest.
Next step would be publishing unstable track GAF files on
pkgs.tailscale.com, with detached signatures, and then making the
clientupdate mechanism also download those and check signatures.
Updates #20002
Change-Id: Ib03c56f17a57f8a4638398ef83549dac4813323d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
772be1b0cc