Adding external subtitle tracks

import { Platform } from 'react-native';
import {
  SourceConfig,
  SourceType,
  SubtitleFormat,
} from 'bitmovin-player-react-native';

// Source config with an external subtitle track.
const config: SourceConfig = {
  url:
    Platform.OS === 'ios'
      ? 'https://bitmovin-a.akamaihd.net/content/sintel/hls/playlist.m3u8'
      : 'https://bitmovin-a.akamaihd.net/content/sintel/sintel.mpd',
  type: Platform.OS === 'ios' ? SourceType.HLS : SourceType.DASH,
  poster: 'https://bitmovin-a.akamaihd.net/content/sintel/poster.png',
  // External subtitle tracks list to be added to this source.
  subtitleTracks: [
    // You can select 'Custom English' in the subtitles menu.
    {
      // The URL of the subtitle file. Required.
      url: 'https://bitdash-a.akamaihd.net/content/sintel/subtitles/subtitles_en.vtt',
      // External file format.
      // Supports `.vtt`, `.ttml` and `.cea` extensions.
      //
      // This option can be left empty since the player automatically recognizes the format
      // from the provided url most of the time.
      format: SubtitleFormat.VTT,
      // Label for this track shown under the selection menu. Required.
      label: 'Custom English',
      // The IETF BCP 47 language tag associated with this track. Required.
      language: 'en',
      // The unique identifier used for this track.
      // The default value for this options is a randomly generated UUID.
      identifier: 'sub1',
      // This track is considered the default if set to `true`.
      // The default value for this option is `false`.
      isDefault: false,
      // If set to `true` it means that the player should automatically select and switch this
      // subtitle according to the selected audio language. Forced subtitles do not appear in
      // `Player.getAvailableSubtitles`.
      //
      // The default value for this option is `false`.
      isForced: false,
    },
    // You may add even more tracks to the list...
  ],
};