FR: Support for ForceAuthn flag in the SAML AuthnRequest

### [REQUIRED] Describe your environment

* Operating System version: macOS 10.15.7
* Browser version: Chrome 86.0.4240.111
* Firebase SDK version: 7.21.1
* Firebase Product: auth

### [REQUIRED] Describe the problem

We want to be able to pass ForceAuthn as an attribute for the AuthnRequest when signing in users with SAML (https://cloud.google.com/identity-platform/docs/web/saml). The goal is to notify the IdP to not use any previous security context when authenticating the user.

As per https://wiki.shibboleth.net/confluence/display/SP3/ForceAuthn:

> SAML includes an optional feature during login requests called ForceAuthn that acts as a signal to the Identity Provider to require some form of user interaction during the course of handling the request, overriding the usual implicit assumption that it's acceptable to reuse authentication state from an earlier request (i.e., Single Sign-On).