cmd/tailscale,ipn: add auto-update flags and prefs (#8861)

The flags are hidden for now. Adding propagation to tailscaled and
persistence only. The prefs field is wrapped in a struct to allow for
future expansion (like update schedule).

Updates #6907

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

ipn/ipn_view.go

@@ -86,6 +86,7 @@ func (v PrefsView) NoSNAT() bool                          { return v.ж.NoSNAT }
 func (v PrefsView) NetfilterMode() preftype.NetfilterMode { return v.ж.NetfilterMode }
 func (v PrefsView) OperatorUser() string                  { return v.ж.OperatorUser }
 func (v PrefsView) ProfileName() string                   { return v.ж.ProfileName }
+func (v PrefsView) AutoUpdate() AutoUpdatePrefs           { return v.ж.AutoUpdate }
 func (v PrefsView) Persist() persist.PersistView          { return v.ж.Persist.View() }
 
 // A compilation failure here means this code must be regenerated, with the command at the top of this file.
@@ -111,6 +112,7 @@ var _PrefsViewNeedsRegeneration = Prefs(struct {
 	NetfilterMode          preftype.NetfilterMode
 	OperatorUser           string
 	ProfileName            string
+	AutoUpdate             AutoUpdatePrefs
 	Persist                *persist.Persist
 }{})