Have a way to have parallel prompt for factors

[brendanheywood]

Not 100% how much we want this but dumping some thoughts.

The use case is when people have multiple factors setup, but only need one of them to get in. We want to give a hybrid UX so they can see a prompt for multiple factors at the same time.

So roughly:

• [ ] the system can already detect whether 1 out of some number of factors is enough, this is the same logic as the 'Summary of conditions
• [ ] we can also filter this list to the factors they actually have setup and require input

At this point the main mfa plugin would simply render two independent forms, and the user can just submit which ever one they want. Some plugins send an email / sms / code to somewhere, but in this scenario it might make more sense to make that an explicit step with a button. But we would not want this when that factor is used in isolation. So we'd either need to touch those factors and add an admin setting for whether we should prompt to send or send immediately. We may also need a new factor method so a factor can opt in / out of this whole parallel idea.