cmd/tailscale/cli: add latest version output to "tailscale version" (#8700)

Add optional `--upstream` flag to `tailscale version` to fetch the
latest upstream release version from `pkgs.tailscale.com`. This is
useful to diagnose `tailscale update` behavior or write other tooling.

Example output:
$ tailscale version --upstream --json
{
	"majorMinorPatch": "1.47.35",
	"short": "1.47.35",
	"long": "1.47.35-t6afffece8",
	"unstableBranch": true,
	"gitCommit": "6afffece8a32509aa7a4dc2972415ec58d8316de",
	"cap": 66,
	"upstream": "1.45.61"
}

Fixes #8669

RELNOTE=adds "tailscale version --upstream"

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

cmd/tailscale/cli/update.go

@@ -874,6 +874,10 @@ func requestedTailscaleVersion(ver, track string) (string, error) {
 	if ver != "" {
 		return ver, nil
 	}
+	return latestTailscaleVersion(track)
+}
+
+func latestTailscaleVersion(track string) (string, error) {
 	url := fmt.Sprintf("https://pkgs.tailscale.com/%s/?mode=json&os=%s", track, runtime.GOOS)
 	res, err := http.Get(url)
 	if err != nil {