Implement custom metadata support

Callback functions are invoked just before the tracking call is made, so your application can attach the metadata that is specific to an ad or chapter.

Invoke callback functions for content, ads, and chapters.

// Video Metadata Block
    vaTrackingMetadata.videoMetadataBlock = ^NSDictionary *()
    {
        return @{
                 @"myvideoid": @"1234",
                 @"mysdkversion": [PTSDK apiVersion]
                 };
    };

// Ad Metadata Block invoked on every ad start
    vaTrackingMetadata.adMetadataBlock = ^NSDictionary *(PTAd *ad)
    {
        return @{
                 @"myadid": @"ad-1234",
                 @"myad-sdkversion": [PTSDK apiVersion]
                 };
    };

// Chapter Metadata Block invoked on every chapter start
    vaTrackingMetadata.chapterMetadataBlock = ^NSDictionary *(PTVideoAnalyticsChapterData *chapter)
    {
        return @{
                 @"mychapterid": @"chapter-1234",
                 @"mychapter-sdkversion": [PTSDK apiVersion]
                 };
    };