gokrazy, clientupdate: add start of Gokrazy auto-updates, tests

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>

clientupdate/clientupdate.go

@@ -11,6 +11,7 @@ import (
 	"bufio"
 	"bytes"
 	"compress/gzip"
+	"context"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -37,6 +38,24 @@ import (
 	"tailscale.com/version/distro"
 )
 
+// GokrazyUpdateArgs contains arguments for updating a Gokrazy appliance from a
+// GAF fetched from a URL.
+type GokrazyUpdateArgs struct {
+	// URL is the GAF download URL.
+	URL string
+
+	// AllowUnsigned permits installing a GAF without signature verification.
+	// This is intended for tests until signed GAF verification is implemented.
+	AllowUnsigned bool
+
+	// Logf is optional; nil discards log messages.
+	Logf logger.Logf
+}
+
+// GokrazyUpdateFromURL updates a Gokrazy appliance from a GAF fetched from a
+// URL, if Gokrazy update support is linked into the binary.
+var GokrazyUpdateFromURL feature.Hook[func(context.Context, GokrazyUpdateArgs) error]
+
 const (
 	StableTrack           = "stable"
 	UnstableTrack         = "unstable"