feat(rdp): RD Gateway transport (experimental)

RDP profiles can go through a Remote Desktop Gateway (MS-TSGU over an
HTTPS WebSocket) with its own username and password, stored like other
credentials (rdp-gateway-password). The gateway stream is bridged to a
loopback socket so the blocking RDP client uses it like any transport.

IronRDP's gateway crate (0.0.1) is vendored with fixes: it used
ironrdp-tls, which accepts any certificate, before sending Basic
credentials — the gateway is now verified against the web PKI; its
asserts on gateway-supplied lengths are errors instead of panics; and
the server port is passed through. Checked against a self-signed
openssl s_server: the connection is refused ('invalid peer certificate')
before any request or credential is sent. Not tested against a real
gateway; Basic-auth gateways only (no NTLM).

rust/vendor/ironrdp-mstsgu/src/macros.rs

@@ -0,0 +1,5 @@
+/// Creates a [`crate::Error`] with `Custom` kind and a source error attached to it
+#[macro_export]
+macro_rules! custom_err {
+    ( $context:expr, $source:expr $(,)? ) => {{ <$crate::Error as $crate::GwErrorExt>::custom($context, $source) }};
+}