Add iOS and Android basic DRM support (#1445)

This PR adds support for DRM streams on iOS (Fairplay) and Android (Playready, Widevine, Clearkey)

I am neither Android nor iOS developer, so feel free to provide feedback to improve this PR.

**Test stream for ANDROID:**
```
testStream = {
        uri: 'http://profficialsite.origin.mediaservices.windows.net/c51358ea-9a5e-4322-8951-897d640fdfd7/tearsofsteel_4k.ism/manifest(format=mpd-time-csf)',
        type: 'mpd',
        drm: {
            type: DRMType.PLAYREADY,
            licenseServer: 'http://test.playready.microsoft.com/service/rightsmanager.asmx?cfg=(persist:false,sl:150)'
        }
    };
```

or 
```
{
    uri: 'https://media.axprod.net/TestVectors/v7-MultiDRM-SingleKey/Manifest_1080p.mpd',
    drm: {
        type: 'widevine', //or DRMType.WIDEVINE
        licenseServer: 'https://drm-widevine-licensing.axtest.net/AcquireLicense',
        headers: {
            'X-AxDRM-Message': 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ2ZXJzaW9uIjoxLCJjb21fa2V5X2lkIjoiYjMzNjRlYjUtNTFmNi00YWUzLThjOTgtMzNjZWQ1ZTMxYzc4IiwibWVzc2FnZSI6eyJ0eXBlIjoiZW50aXRsZW1lbnRfbWVzc2FnZSIsImZpcnN0X3BsYXlfZXhwaXJhdGlvbiI6NjAsInBsYXlyZWFkeSI6eyJyZWFsX3RpbWVfZXhwaXJhdGlvbiI6dHJ1ZX0sImtleXMiOlt7ImlkIjoiOWViNDA1MGQtZTQ0Yi00ODAyLTkzMmUtMjdkNzUwODNlMjY2IiwiZW5jcnlwdGVkX2tleSI6ImxLM09qSExZVzI0Y3Iya3RSNzRmbnc9PSJ9XX19.FAbIiPxX8BHi9RwfzD7Yn-wugU19ghrkBFKsaCPrZmU'
        },
    }
}
```

**Test stream for iOS:**
Sorry but I can not provide free streams to test. If anyone can provide test streams, or found some we can use, please let me know to also test them.

It has been tested with a private provider and they work, at least with the `getLicense` override method. (An example implementation is provided in the README)