feat(auth,ui): support github sign-in in the UI (#2545)

# What does this PR do?
Uses NextAuth to add github sign in support.

## Test Plan
Start server with auth configured as in
https://github.com/meta-llama/llama-stack/pull/2509


https://github.com/user-attachments/assets/61ff7442-f601-4b39-8686-5d0afb3b45ac

llama_stack/ui/instrumentation.ts

@@ -0,0 +1,11 @@
+/**
+ * Next.js Instrumentation
+ * This file is used for initializing monitoring, tracing, or other observability tools.
+ * It runs once when the server starts, before any application code.
+ *
+ * Learn more: https://nextjs.org/docs/app/building-your-application/optimizing/instrumentation
+ */
+
+export async function register() {
+  await import("./lib/config-validator");
+}