Add the ability to edit remote directories over SSH (#14530)

This is a first step towards allowing you to edit remote projects
directly over SSH. We'll start with a pretty bare-bones feature set, and
incrementally add further features.

### Todo

Distribution
* [x] Build nightly releases of `zed-remote-server` binaries
    * [x] linux (arm + x86)
    * [x] mac (arm + x86)
* [x] Build stable + preview releases of `zed-remote-server`
* [x] download and cache remote server binaries as needed when opening
ssh project
* [x] ensure server has the latest version of the binary


Auth
* [x] allow specifying password at the command line
* [x] auth via ssh keys
* [x] UI password prompt

Features
* [x] upload remote server binary to server automatically
* [x] opening directories
* [x] tracking file system updates
* [x] opening, editing, saving buffers
* [ ] file operations (rename, delete, create)
* [ ] git diffs
* [ ] project search

Release Notes:

- N/A

---------

Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>

crates/gpui/src/app.rs

@@ -112,7 +112,18 @@ impl App {
         log::info!("GPUI was compiled in test mode");
 
         Self(AppContext::new(
-            current_platform(),
+            current_platform(false),
+            Arc::new(()),
+            http::client(None),
+        ))
+    }
+
+    /// Build an app in headless mode. This prevents opening windows,
+    /// but makes it possible to run an application in an context like
+    /// SSH, where GUI applications are not allowed.
+    pub fn headless() -> Self {
+        Self(AppContext::new(
+            current_platform(true),
             Arc::new(()),
             http::client(None),
         ))