Support contextual presentation requests

Currently presentation requests are statically defined and sent to the wallet/user in the auth request. This seems inefficient in the case where the user is re-authenticating using SIOPv2 and has already submitted/fulfilled the presentation request. They may have to submit the same presentation each time they authenticate.

Instead, it would be nice to support a flow like:

1. User authenticates
2. Server contextually determines whether to send a new presentation request or not before proceeding
3. [Optional] User submits presentation response
4. Auth token sent to user

I believe this means updating this section. I'm not sure how to fit this in to the current flow and could use some feedback from the group. Perhaps a user gets a preliminary access token and after presentation they get a new access token with increased scope.