mux: ssh: add override_proxy_command to SshDomain

This enables users to override the normal invocation completely,
which is helpful for example when connecting into a gui from
a remote host.

I was wondering why prefer-mux was in here, and my commit message
from 7afc9e4d56 suggests that I
hit an issue where the gui wasn't running and where I didn't care
because I wanted to hit the mux.

I think that makes it a little awkward to blanket remove the
--prefer-mux flag, but for those that don't want it, they should now
be able to configure to allow connected to the gui

```
  ssh_domains = {
    {
      name = "shortname",
      remote_address = "hostname",
      override_proxy_command = "wezterm cli proxy",
    },
  },
```

config/src/ssh.rs

@@ -83,6 +83,9 @@ pub struct SshDomain {
 
     /// The path to the wezterm binary on the remote host
     pub remote_wezterm_path: Option<String>,
+    /// Override the entire `wezterm cli proxy` invocation that would otherwise
+    /// be computed from remote_wezterm_path and other information.
+    pub override_proxy_command: Option<String>,
 
     pub ssh_backend: Option<SshBackend>,
 

wezterm-client/src/client.rs

@@ -664,7 +664,9 @@ impl Reconnectable {
         let sess = ssh_connect_with_ui(ssh_config, ui)?;
         let proxy_bin = Self::wezterm_bin_path(&ssh_dom.remote_wezterm_path);
 
-        let cmd = if initial {
+        let cmd = if let Some(cmd) = ssh_dom.override_proxy_command.clone() {
+            cmd
+        } else if initial {
             format!("{} cli --prefer-mux proxy", proxy_bin)
         } else {
             format!("{} cli --prefer-mux --no-auto-start proxy", proxy_bin)