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/StatusBar.axaml

@@ -9,8 +9,9 @@
 
     The design puts five things here: the active host and the bastion it is reached through, the open port
     forwards, the SFTP transfer count, an auto-lock countdown, and the last sync time. Four of those five
-    have nothing behind them — there is no jump-host routing, no port forwarding, no file transfer and no
-    idle auto-lock — so what is left is the active session, the vault's last message, and the shortcut hint.
+    had nothing behind them when this was written — there was no jump-host routing, no port forwarding, no
+    file transfer and no idle auto-lock — so what is left is the active session, the vault's last message,
+    and the shortcut hint. Forwards have since been built, and are counted on SessionStatusBar instead.
 
     The last message is the load-bearing one, and it is why this bar is worth having at all: the vault's
     status line used to be a cramped label in the account bar, and everything the application has to say