Add PlaybackSpeed into Media.Played (COVESA#499)

* Add PlaybackSpeed into Media.Played

- It needs to add Playback Speed into Media.Played branch because it's important feature to change playback speed on some media services as audio book, youtube video

* the restrictions of PlaybackSpeed

the restrictions of PlaybackSpeed

* Additional descriptions for meaning of the values

Add text below for better understanding. 
 comment: 1 means normal speed.
           <1 means slower playback than normal.
           >1 means faster playback than normal.

* Revised with the meeting result

- Aligned work with https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/playbackRate

spec/Cabin/Infotainment.vspec

@@ -49,6 +49,14 @@ Media.Played.URI:
   type: sensor
   description: User Resource associated with the media
 
+Media.Played.PlaybackRate:
+  datatype: float
+  type: actuator
+  description: Current playback rate of media being played.
+  comment: The normal playback rate is multiplied by this value to obtain the current rate, so a value of 1.0 indicates normal speed.
+           Values of lower than 1.0 make the media play slower than normal.
+           Values of higher than 1.0 make the media play faster than normal.
+
 Media.DeclinedURI:
   datatype: string
   type: sensor