ipn: introduce app connector advertisement preference and flags

Introduce a preference structure to store the setting for app connector
advertisement.

Introduce the associated flags:

  tailscale up --advertise-connector{=true,=false}
  tailscale set --advertise-connector{=true,=false}

```
% tailscale set --advertise-connector=false
% tailscale debug prefs | jq .AppConnector.Advertise
false
% tailscale set --advertise-connector=true
% tailscale debug prefs | jq .AppConnector.Advertise
true
% tailscale up --advertise-connector=false
% tailscale debug prefs | jq .AppConnector.Advertise
false
% tailscale up --advertise-connector=true
% tailscale debug prefs | jq .AppConnector.Advertise
true
```

Updates tailscale/corp#15437

Signed-off-by: James Tucker <james@tailscale.com>

ipn/ipn_view.go

@@ -88,6 +88,7 @@ func (v PrefsView) NetfilterMode() preftype.NetfilterMode { return v.ж.Netfilte
 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) AppConnector() AppConnectorPrefs       { return v.ж.AppConnector }
 func (v PrefsView) PostureChecking() bool                 { return v.ж.PostureChecking }
 func (v PrefsView) Persist() persist.PersistView          { return v.ж.Persist.View() }
 
@@ -116,6 +117,7 @@ var _PrefsViewNeedsRegeneration = Prefs(struct {
 	OperatorUser           string
 	ProfileName            string
 	AutoUpdate             AutoUpdatePrefs
+	AppConnector           AppConnectorPrefs
 	PostureChecking        bool
 	Persist                *persist.Persist
 }{})