Wait for extension event to complete

[janfejtek]

Hi, I would like to get microsoft token using MSAL library and then send it to server, but the extension does not wait for events to finish. Can this be somehow achieved? I can create PR but I'm not sure if it makes sense for the library or how the solution should look like.

Simplified version of my usage would be:

export class TokenExtension {

    initialize(naja) {
        naja.addEventListener('before', this.before.bind(this));
    }
    async before(event) {
       let token = await fetch('https://microsoft.com');
       event.request.headers.set('X-Token', token);
    }
}

I noticed similar point here https://github.com/naja-js/naja/issues/383 . Unfortunately, I'm too late for 3.0 release...