Notify user when DMing a guest, based on setting

This is the zulip-flutter side of the following upcoming Zulip feature, with corresponding server/web and zulip-mobile issues:

• https://github.com/zulip/zulip/issues/30078
• https://github.com/zulip/zulip-mobile/issues/5872

The feature is probably landing in Zulip Server 9.0, so it'd be good to implement soon. That means:

• consume that new setting from the server;
• when appropriate, show a banner above the compose box similar to the one shown in Zulip web and zulip-mobile.

References

• Design thread: #mobile-design > DMing a guest @ 💬
• Server/web issue: https://github.com/zulip/zulip/issues/30078
• Similar feature but not directly related at a code level: #791