Expose Fetcher HTTP Responses to user via event

[alexgdiaz98]

@katspaugh I would like to expose the raw HTTP responses handled by Fetcher to the user by emitting a new event type. My file server embeds useful file metadata in the response headers. Do you think this is a good idea?

Ex:

// wavesurfer.js:
private async loadAudio(url: string, blob?: Blob, channelData?: WaveSurferOptions['peaks'], duration?: number) {
    ...
    // Fetch the entire audio as a blob if pre-decoded data is not provided
    if (!blob && !channelData) {
      const onProgress = (percentage: number) => this.emit('loading', percentage)
      const onResponse = (response: Response) => this.emit('response', response)
      blob = await Fetcher.fetchBlob(url, onProgress, onResponse, this.options.fetchParams)
    }
    ...
}

// fetcher.js:
async function fetchBlob(
  url: string,
  progressCallback: (percentage: number) => void,
  responseCallback: (response: Response) => void,
  requestInit?: RequestInit,
): Promise<Blob> {
  // Fetch the resource
  const response = await fetch(url, requestInit)
  responseCallback(response)

  if (!response.ok) {
    throw new Error(`Failed to fetch ${url}: ${response.status} (${response.statusText})`)
  }

  // Read the data to track progress
  watchProgress(response.clone(), progressCallback)

  return response.blob()
}

[katspaugh]

I think 99% of the users won't need it, so I wouldn't complicate the API. Custom problems require custom solutions. So I would recommend fetching audio in your app code and calling wavesurfer.loadBlob to load it into wavesurfer.