extension/dap: Add resolve_tcp_template function (#31010)

Extensions cannot look up available port themselves, hence the new API.
With this I'm able to port our Ruby implementation into an extension.

Release Notes:

- N/A

crates/dap/src/registry.rs

@@ -54,10 +54,6 @@ impl DapRegistry {
     pub fn add_adapter(&self, adapter: Arc<dyn DebugAdapter>) {
         let name = adapter.name();
         let _previous_value = self.0.write().adapters.insert(name, adapter);
-        debug_assert!(
-            _previous_value.is_none(),
-            "Attempted to insert a new debug adapter when one is already registered"
-        );
     }
 
     pub fn adapter_language(&self, adapter_name: &str) -> Option<LanguageName> {