Add volume property to Navigation branch (COVESA#504)

* Add volume property to Navigation branch

It need to have a volume property of Navigation to control the navigation guidance volume.

* add unit: percent  

-  Added unit: percent to Navigation.Volume and Media.Volume

spec/Cabin/Infotainment.vspec

@@ -72,6 +72,7 @@ Media.Volume:
   type: actuator
   min: 0
   max: 100
+  unit: percent  
   description: Current Media Volume
 
 Navigation:
@@ -104,6 +105,14 @@ Navigation.Mute:
   allowed: ['MUTED', 'ALERT_ONLY', 'UNMUTED']
   description: Navigation mute state that was selected.  
 
+Navigation.Volume:
+  datatype: uint8
+  type: actuator
+  min: 0
+  max: 100
+  unit: percent    
+  description: Current navigation volume
+
 HMI:
   type: branch
   description: HMI related signals