feat: allow relative redirects (#3140)

Author: balazsorban44

src/core/lib/default-callbacks.ts

@@ -6,6 +6,7 @@ export const defaultCallbacks: CallbacksOptions = {
   },
   redirect({ url, baseUrl }) {
     if (url.startsWith(baseUrl)) return url
+    else if (url.startsWith("/")) return new URL(url, baseUrl).toString()
     return baseUrl
   },
   session({ session }) {