feat: loadUrl adds headers parameter

[biner88]

Description

Some playback links require header authentication

Requirements

• [ ] add headers

Additional Context

Future<AudioSource> loadUrl(
    String url,
    LoadMode mode, {
    http.Client? httpClient,
    Map<String, String>? headers,
  })

[misterfourtytwo]

you should be able to provide headers through http.Client configuration, nope?