ffi: AuthenticationService treats sliding sync proxy as mandatory

Calling [`configure_homeserver`](https://github.com/matrix-org/matrix-rust-sdk/blob/main/bindings/matrix-sdk-ffi/src/authentication_service.rs#L320) on an `AuthenticationService` instance currently results in an error if there is neither a custom sliding sync proxy nor a sliding sync proxy in the home server's .well-known. It would be great if login was possible on home servers that don't currently support sliding sync. Maybe this could be achieved through an additional parameter in the constructor of `AuthenticationService`?

Start in bindings/matrix-sdk-ffi/src/authentication_service.rs at configure_homeserver and inspect how the AuthenticationService constructor and home server .well-known data determine the sliding sync proxy. Trace the existing error path and related authentication tests; done means login can proceed for a home server without sliding sync support while proxy configuration still works.

Written by the indexing model from the issue text.