- SoundstripeAudioProvider support added

[luis901101]

This PR adds support to audioProviders on Configuration options, specifically supporting SoundstripeAudioProvider. Since Soundstripe integration support was added to Android and iOS native libraries back on Feb 16, I was expecting an update on flutter plugin supporting these integrations.

I understand Audio Providers support from flutter side is not an easy feature and I also understand the way I added this support could not be the ideal way, as I only modified code on imgly-flutter plugin code to support Soundstripe and maybe this integration should imply more modules to support other Audio Providers.

Anyway I share the code so you can check it and decide if it works for a merge as a first approach until full audio provider support can be added to flutter.

Usage would be as simple as

final configuration = Configuration(
      audioProviders: [
        SoundstripeAudioProvider(
            'https://api.soundstripe.myproxy.com',
            headers: {"Authorization": "Token XXXXXX"}
        ),
      ],
    );