ipn,tailcfg: add app connector config knob to conffile (#13942)

Make it possible to advertise app connector via a new conffile field.
Also bumps capver - conffile deserialization errors out if unknonw
fields are set, so we need to know which clients understand the new field.

Updates tailscale/tailscale#11113

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

ipn/conf.go

@@ -32,6 +32,8 @@ type ConfigVAlpha struct {
 	AdvertiseRoutes []netip.Prefix `json:",omitempty"`
 	DisableSNAT     opt.Bool       `json:",omitempty"`
 
+	AppConnector *AppConnectorPrefs `json:",omitempty"` // advertise app connector; defaults to false (if nil or explicitly set to false)
+
 	NetfilterMode       *string  `json:",omitempty"` // "on", "off", "nodivert"
 	NoStatefulFiltering opt.Bool `json:",omitempty"`
 
@@ -137,5 +139,9 @@ func (c *ConfigVAlpha) ToPrefs() (MaskedPrefs, error) {
 		mp.AutoUpdate = *c.AutoUpdate
 		mp.AutoUpdateSet = AutoUpdatePrefsMask{ApplySet: true, CheckSet: true}
 	}
+	if c.AppConnector != nil {
+		mp.AppConnector = *c.AppConnector
+		mp.AppConnectorSet = true
+	}
 	return mp, nil
 }