VLCMediaMetaData: add meta extra related API

VLCMediaMetaData: add meta extra related API

It works with hash from vlc master (dfe555a96c8cbbdad1b55c090486936b640268b6)

Objective-C

VLCMedia *media = ...;
VLCMediaMetaData * metaData = media.metaData;
// get all meta extra
NSDictionary<NSString *, NSString *> *extra = metaData.extra;
// get specific value
NSString *value = [metaData extraValueForKey: @"KEY"];
// set meta extra value
[metaData setExtraValue: @"VALUE" forKey: @"KEY"];

Swift

let media: VLCMedia = ...
let metaData: VLCMedia.MetaData = media.metaData
// get all meta extra
let extra: [String : String]? = metaData.extra
// get specific value
let value: String? = metaData.extraValue(forKey: "KEY")
// set meta extra value
metaData.setExtraValue("VALUE", forKey: "KEY")

I think you can improve the user experience by using VLC iOS to display lyrics, etc.

Create a draft as an idea.