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/php.rs

@@ -101,7 +101,7 @@ impl PhpDebugAdapter {
 
 #[async_trait(?Send)]
 impl DebugAdapter for PhpDebugAdapter {
-    fn dap_schema(&self) -> serde_json::Value {
+    async fn dap_schema(&self) -> serde_json::Value {
         json!({
             "properties": {
                 "request": {