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/src/adapters.rs

@@ -386,7 +386,7 @@ pub trait DebugAdapter: 'static + Send + Sync {
         }
     }
 
-    fn dap_schema(&self) -> serde_json::Value;
+    async fn dap_schema(&self) -> serde_json::Value;
 }
 
 #[cfg(any(test, feature = "test-support"))]
@@ -434,7 +434,7 @@ impl DebugAdapter for FakeAdapter {
         DebugAdapterName(Self::ADAPTER_NAME.into())
     }
 
-    fn dap_schema(&self) -> serde_json::Value {
+    async fn dap_schema(&self) -> serde_json::Value {
         serde_json::Value::Null
     }