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/src/adapters.rs

@@ -181,7 +181,7 @@ impl DebugTaskDefinition {
 /// Created from a [DebugTaskDefinition], this struct describes how to spawn the debugger to create a previously-configured debug session.
 #[derive(Debug, Clone, PartialEq)]
 pub struct DebugAdapterBinary {
-    pub command: String,
+    pub command: Option<String>,
     pub arguments: Vec<String>,
     pub envs: HashMap<String, String>,
     pub cwd: Option<PathBuf>,
@@ -437,7 +437,7 @@ impl DebugAdapter for FakeAdapter {
         _: &mut AsyncApp,
     ) -> Result<DebugAdapterBinary> {
         Ok(DebugAdapterBinary {
-            command: "command".into(),
+            command: Some("command".into()),
             arguments: vec![],
             connection: None,
             envs: HashMap::default(),