Refactor ServeSession to support oneshot sessions

- Updated the ServeSession struct to allow for optional ChangeProcessor and related fields, enabling lightweight oneshot sessions that do not require live filesystem monitoring.
- Modified the new and new_oneshot methods to initialize sessions accordingly, improving flexibility for commands that only need a snapshot of the tree.
- Adjusted related CLI commands to utilize the new oneshot session functionality, enhancing performance for operations like plugin installation and syncback.

These changes streamline session management and improve the efficiency of operations that do not require continuous updates.

src/cli/plugin.rs

@@ -55,7 +55,7 @@ fn initialize_plugin() -> anyhow::Result<ServeSession> {
     in_memory_fs.load_snapshot("/plugin", plugin_snapshot)?;
 
     let vfs = Vfs::new(in_memory_fs);
-    Ok(ServeSession::new(vfs, "/plugin", None)?)
+    Ok(ServeSession::new_oneshot(vfs, "/plugin")?)
 }
 
 fn install_plugin() -> anyhow::Result<()> {