Feature Request: Set preferred authentication method

`ement-connect` hardcodes a completing-read for the authentication method in the `flows-callback` label. I'd like to be able to set a preferred authentication method to avoid the additional prompt when connecting.

I can imagine implementing a user option for this several ways depending on how flexible you want it to be. The simplest would just be a symbol indicating the preferred method. e.g.

```emacs-lisp
(defcustom ement-preferred-authentication-method 'password)
```

Slightly more complex would be allowing a preference per homeserver or arbitrary predicate matching.