Add unified search toggle in config

The new global search UI is new and might be unstable, hence
we are giving users the option to use the old unified search UI, if
the encounter signficant bottlenecks.

Signed-off-by: fenn-cs <fenn25.fn@gmail.com>
Signed-off-by: nextcloud-command <nextcloud-command@users.noreply.github.com>

config/config.sample.php

@@ -2395,4 +2395,11 @@ $CONFIG = [
  * Defaults to ``true``
  */
 'reference_opengraph' => true,
+
+/**
+ * Enable use of old unified search
+ *
+ * Defaults to ``false``
+ */
+'unified_search.enabled' => false,
 ];