Add port forwarding: local, remote and dynamic, per terminal session

A host can now carry port forwards (-L, -R, -D), saved inside its
encrypted payload as RelayTicketRequest.PortForwardId already assumed,
so they sync and merge per forward like tags and pinned paths. The host
schema moves to version 8 only for hosts that carry forwards.

ISshConnection gains StartForwardAsync over SSH.NET's forwarded ports.
Every listener binds loopback and there is no bind-address field: a
forward is something any member of a shared keychain can edit. A remote
forward's target is resolved here, IPv4 first, rather than by SSH.NET's
constructor, which takes the first DNS answer and can send ::1.

Forwards start with every terminal opened to the host and stop with its
connection; a reconnect starts them again. The session sidebar lists
them with state, failure reason and an on/off switch, the status bar
counts the listening ones, and the host editor and detail pane gain a
PORT FORWARDS section. A forward that cannot start fails alone.

The phone does not start forwards yet: it has no surface showing what
is listening, and its More screen now says so. No byte counts, since
SSH.NET keeps none.

src/DodoSSH.Client.App/Views/SessionSidebar.axaml.cs

@@ -19,6 +19,10 @@ internal sealed partial class SessionSidebar : UserControl
     internal static readonly StyledProperty<bool> ShowsSnipsProperty =
         AvaloniaProperty.Register<SessionSidebar, bool>(nameof(ShowsSnips));
 
+    /// <summary>Whether the FORWARDS section draws. The terminal's own, for the reason SNIPS is.</summary>
+    internal static readonly StyledProperty<bool> ShowsForwardsProperty =
+        AvaloniaProperty.Register<SessionSidebar, bool>(nameof(ShowsForwards));
+
     public SessionSidebar() => InitializeComponent();
 
     internal bool ShowsSnips
@@ -26,4 +30,10 @@ internal sealed partial class SessionSidebar : UserControl
         get => GetValue(ShowsSnipsProperty);
         set => SetValue(ShowsSnipsProperty, value);
     }
+
+    internal bool ShowsForwards
+    {
+        get => GetValue(ShowsForwardsProperty);
+        set => SetValue(ShowsForwardsProperty, value);
+    }
 }