debugger: Remove fake adapter and un-gate GDB (#27557)

This is a clean-up PR in anticipation of introduction of Debugger
Registry. I wanna get rid of DebugAdapterKind (or rather, it being an
enum).
Release Notes:

- N/A

---------

Co-authored-by: Anthony Eid <hello@anthonyeid.me>
Co-authored-by: Anthony <anthony@zed.dev>

crates/dap/src/registry.rs

@@ -0,0 +1,39 @@
+use parking_lot::RwLock;
+
+use crate::adapters::{DebugAdapter, DebugAdapterName};
+use std::{collections::BTreeMap, sync::Arc};
+
+#[derive(Default)]
+struct DapRegistryState {
+    adapters: BTreeMap<DebugAdapterName, Arc<dyn DebugAdapter>>,
+}
+
+#[derive(Default)]
+/// Stores available debug adapters.
+pub struct DapRegistry(Arc<RwLock<DapRegistryState>>);
+
+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(&self, name: &str) -> Option<Arc<dyn DebugAdapter>> {
+        self.0.read().adapters.get(name).cloned()
+    }
+    pub fn enumerate_adapters(&self) -> Vec<DebugAdapterName> {
+        self.0.read().adapters.keys().cloned().collect()
+    }
+
+    #[cfg(any(test, feature = "test-support"))]
+    pub fn fake() -> Self {
+        use crate::FakeAdapter;
+
+        let register = Self::default();
+        register.add_adapter(Arc::new(FakeAdapter::new()));
+        register
+    }
+}