feat: allow plain domain in dcaccount: scheme

This is similar to old `dcaccount:` with URL,
but creates a 9-character username on the client
and avoids making an HTTPS request.

The scheme is reused to avoid the apps
needing to register for the new scheme.

`http` support is removed because it was
not working already, there is a check
that the scheme is `https` when the URL
is actually used and the core has
no way to make HTTP requests without TLS.

deltachat-rpc-client/tests/test_something.py

@@ -816,7 +816,7 @@ def test_configured_imap_certificate_checks(acfactory):
     alice = acfactory.new_configured_account()
 
     # Certificate checks should be configured (not None)
-    assert "cert_automatic" in alice.get_info().used_account_settings
+    assert "cert_strict" in alice.get_info().used_account_settings
 
     # "cert_old_automatic" is the value old Delta Chat core versions used
     # to mean user entered "imap_certificate_checks=0" (Automatic)