Add staged checkboxes to multibuffer headers (#24308)

Co-authored-by: Mikayla <mikayla@zed.dev>

Release Notes:

- N/A

---------

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/editor/src/editor.rs

@@ -124,7 +124,8 @@ pub use multi_buffer::{
     ToOffset, ToPoint,
 };
 use multi_buffer::{
-    ExpandExcerptDirection, MultiBufferDiffHunk, MultiBufferPoint, MultiBufferRow, ToOffsetUtf16,
+    ExcerptInfo, ExpandExcerptDirection, MultiBufferDiffHunk, MultiBufferPoint, MultiBufferRow,
+    ToOffsetUtf16,
 };
 use project::{
     lsp_store::{FormatTrigger, LspFormatTarget, OpenLspBufferHandle},
@@ -580,6 +581,15 @@ struct BufferOffset(usize);
 pub trait Addon: 'static {
     fn extend_key_context(&self, _: &mut KeyContext, _: &App) {}
 
+    fn render_buffer_header_controls(
+        &self,
+        _: &ExcerptInfo,
+        _: &Window,
+        _: &App,
+    ) -> Option<AnyElement> {
+        None
+    }
+
     fn to_any(&self) -> &dyn std::any::Any;
 }