feat(mediaComposition): exposes the media URN through the main resources

Today, it's not possible to retrieve the identifier of the media currently being
 played. In some cases, however, this information may be of interest to the
 developer. This modification fills the gap by exposing the URN through the
 `mediaData` object accessible from the player current source.

 ```javascript
 // Access the media URN
 player.currentSource().mediaData.urn
 ```

- add `urn` property to `mediaComposition.getMainResources`
- update `getMainResources` test case to ensure that the urn property is exposed

src/dataProvider/model/MediaComposition.js

@@ -202,6 +202,7 @@ class MediaComposition {
       streamOffset: resource.streamOffset,
       tokenType: resource.tokenType,
       url: resource.url,
+      urn: this.chapterUrn
     }));
   }
 

test/dataProvider/model/MediaComposition.spec.js

@@ -519,6 +519,7 @@ describe('MediaComposition', () => {
       expect(resource).toHaveProperty('streamOffset');
       expect(resource).toHaveProperty('tokenType');
       expect(resource).toHaveProperty('url');
+      expect(resource).toHaveProperty('urn');
     });
 
     it('should return the main resources for an on demand video', () => {