Add a fake SAML handler/form to fake-gp-server.py

Add a fake SAML handler/form to fake-gp-server.py

This allows authenticating to the fake server with https://github.com/dlenski/gp-saml-gui

    # Start fake server
    $ ./fake-gp-server localhost 8080 certs/server-{cert,key}.pem 2>&1 >/dev/null &

    # Configure fake server for SAML on the portal interface
    $ curl -sk https://localhost:8080/CONFIGURE -d portal_saml=portal-userauthcookie -d portal_cookie=portal-userauthcookie

    # Use gp-saml-gui to authenticate to it
    $ gp-saml-gui --no-verify localhost:8080
    ...
    ... pops up window
    ... fills out login form
    ...
    HOST=https://localhost:8080/global-protect/getconfig.esp:portal-userauthcookie
    USER=nobody
    COOKIE=FAKE_username_nobody_password_whatever
    OS=linux-64

The goal of this is to have a SAML-supporting GP server to test against
while modifying openconnect to directly call the GP SAML webview handler
itself (see https://github.com/dlenski/gp-saml-gui/issues/45).

Signed-off-by: Daniel Lenski <dlenski@gmail.com>