Add optional callback params to record-video extension (for data handling)

[becky-gilbert]

The record-video extension saves the data as a base64 string in the jsPsych, and there doesn't seem to be a way to intercept the video blob data before that point. Base64 strings are a useful way of storing non-text data as strings so that it is available to researchers via the jsPsych data, but it's also non-ideal in many cases (for instance because video recordings can quickly generate a huge amount of data).

It would be really useful if the record-video extension provided some optional function callback parameters that would get called during specific extension events (initialization, on_start, on_finish, etc.) and, when video data is available, receive it as an argument. This would allow researchers to send the video .webm files directly to a server.

If these parameters made available at the extension initialization, then it could look something like this:

const start_custom_upload = () => {
  // set up upload
};

const finish_custom_upload = (videoBlob) => {
  // handle data
};


let jsPsych = initJsPsych({
  extensions: [
    {
        type: jsPsychExtensionRecordVideo, 
        params: {
            on_start: start_custom_upload, 
            on_finish: finish_custom_upload
        }
    }
  ]
});

For naming these extension parameters, it might make sense to use the extension event names (on_start etc.) and/or the media recorder API event names (ondataavailable, etc.).

Remember to add the following in the PR:

• changelog file
• change to docs (in docs/extensions/record-video.md)
• minimal example (on docs page and/or in examples folder)

cc @okaycj