debugger: Mark DebugAdapterBinary::program as optional (#32534)

This allows us to support debugging with a debug adapter not managed by
Zed. Note that this is not a user facing change, as DebugAdapterBinary
is used to determine how to spawn a debugger. Thus, this should not
break any configs or anything like that.

Closes #ISSUE

Release Notes:

- N/A

crates/collab/src/tests/remote_editing_collaboration_tests.rs

@@ -671,7 +671,7 @@ async fn test_remote_server_debugger(
     });
 
     session.update(cx_a, |session, _| {
-        assert_eq!(session.binary().command, "ssh");
+        assert_eq!(session.binary().command.as_deref(), Some("ssh"));
     });
 
     let shutdown_session = workspace.update(cx_a, |workspace, cx| {