title: saml2 idp authentication forwarding
body: <p>I have an app that authenticate against an idP using SAML2.
  The app support only one idP.<br>
  The app is multi-tenant, and I need to forward each tenant user to their own idP.<br>
  So I need my local idP (MyIdP, that is registered in the app) to ask the user for their tenat name and based on that, forward the authentication to the appropiate idP and then return it result to the app as its result.<br>
  Something like a idP proxy (Is there a name for this functionality???)<br>
  Is it possible to do it using SimpleSAMLphp, Shibboleth, Keycloak or any other open source idP?</p>
  <p>(App)-----&gt;(MyIdP)-----------------------------(idP-Tenant-A)<br>
                         |                                          |-----(idP-Tenant-B)<br>
         { Ask User Tenant Name                    |-----(idP-Tenant-C)<br>
          and forward based on it }</p>
  <p>Thanks</p>