Add SAML/SSO login capability
Fractal supports only username/password authentication. As those don't exist side by side an SAML-enabled synapse instance can't be used with Fractal.
Riot already supports SAML-based setups. Some details on how to configure keycloak (as SAML reference) with synapse can be found here: https://edenmal.moe/post/2019/Matrix-Synapse-SAML2-Login/
Matrix protocol standard for SSO logins can be found here: https://matrix.org/docs/spec/client_server/r0.5.0#sso-client-login
(Feel free to add the whole tasks, mocks,etc to this issue. I'm not aware enough about fractal internals to make an assumption on that.)