Custom callback for an existing strategy.

[daveli]

👋 First of all thank you for creating Pow + Assent!

I would like to get some guidance on how to tackle the following scenario. I'm currently working on integrating pow_assent in a Chrome extension. This requires some "special" handling when it comes to the redirect_uri looking like this:

• Launching the auth window in Chrome (chrome.identity.launchWebAuthFlow) with the following URL http://localhost:4000/auth/{provider}/new?session_params={state:{callback_url: callback_url provided by Chrome }}.
• Doing the whole flow and returning to http://localhost:4000/auth/{provider}/callback

Now I would like to control just the callback part for this provider as I need to extract the callback_url from the session_params[state] append the token and use that as the redirect URL. Any pointers on how to approach this? Once I have figured this out I will write a guide and send this is as a PR for others wanting to do the same.

Cheers!