title: IdentityServer4 client_id specific login pages
body: <p>I'm trying to have different login pages based on the client_id.</p>
  
  <p>Use case :
  My default login page is a classic username/password type login, but for a specific client_id, the login page asks for 3 different infos that are found one a piece of paper that he received in the mail (sent by a third party).
  Once i have these 3 infos, i can validate and find the associated user. </p>
  
  <p>Technicals : So far, i have made it so that once IdentityServer4 redirects /connect/authorize to it's default login route (/account/login), i then redirect to my second login based on the client_id. It works but it is not elegant at all (feels hackish).
  I'm sure there is a better way to achieve this, probably thru a middleware that would redirect from connect/authorize to my second login page, directly ?</p>
  
  <p>Any ideas / tips ? </p>