Add options to configure SAML connection in the role.

Linked to https://forum.laquadrature.net/t/sso-md-lqdn-fr/222

And https://git.laquadrature.net/lqdn-interne/equipe_technique/-/issues/14

The documentation is here ; https://docs.hedgedoc.org/configuration/#saml-login

• Add variables for configuring the role
• Add installation option to take into account the SSO config
• Add tests