feat: support underscore-prefixed domains with self-signed TLS certificates

Allow Delta Chat core to work with chatmail servers running on
underscore-prefixed domains (e.g. _alice.localchat) which use
self-signed TLS certificates. This is mirroring related work
on chatmail relays: https://github.com/chatmail/relay/pull/855
Underscore domains with self-signed TLS certs can be used by LXC test
containers where obtaining real certificates is not practical.

When the domain starts with '_', certificate verification is
automatically relaxed for IMAP/SMTP connections, dcaccount QR
code handling, and iroh relay endpoints. The Python test suite
is adapted to also work against such underscore-domain servers,
including cross-core tests with older Delta Chat versions.

Note: this PR does not support HTTPS requests with underscore
domains. They are not currently needed for working with LXC test
containers.

14 files changed, +102/-31 lines (excluding Cargo.lock).
Cargo.lock: +606/-11 lines from enabling iroh features
needed for connecting to iroh relay endpoint on underscore domains.
The added dependencies are unfortunate but best considered
when finally upgrading to iroh 1.0 (tm).

deltachat-rpc-client/tests/test_securejoin.py

@@ -141,10 +141,9 @@ def test_qr_securejoin_broadcast(acfactory, all_devices_online):
 
     def wait_for_broadcast_messages(ac):
         snapshot1 = ac.wait_for_incoming_msg().get_snapshot()
-        assert snapshot1.text == "You joined the channel."
-
         snapshot2 = ac.wait_for_incoming_msg().get_snapshot()
-        assert snapshot2.text == "Hello everyone!"
+        texts = {snapshot1.text, snapshot2.text}
+        assert texts == {"You joined the channel.", "Hello everyone!"}
 
         chat = get_broadcast(ac)
         assert snapshot1.chat_id == chat.id