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/dap_adapters/src/ruby.rs

@@ -175,7 +175,7 @@ impl DebugAdapter for RubyDebugAdapter {
         arguments.extend(ruby_config.args);
 
         Ok(DebugAdapterBinary {
-            command: rdbg_path.to_string_lossy().to_string(),
+            command: Some(rdbg_path.to_string_lossy().to_string()),
             arguments,
             connection: Some(dap::adapters::TcpArguments {
                 host,