debugger beta: Fix dap_schema for DAP extensions (#31173)

We now actually call dap_schema provided by extensions instead of
defaulting to a null `serde_json::Value`. We still need to update the
Json LSP whenever a new dap is installed.

Release Notes:

- N/A

crates/dap_adapters/src/javascript.rs

@@ -170,7 +170,7 @@ impl DebugAdapter for JsDebugAdapter {
         })
     }
 
-    fn dap_schema(&self) -> serde_json::Value {
+    async fn dap_schema(&self) -> serde_json::Value {
         json!({
             "oneOf": [
                 {