[wip] search-backend: result-by-result authorization

To do:
- add tests for AuthorizedSearchEngine
- update router tests
- add diff to changeset

Signed-off-by: MT Lewis <mtlewis@users.noreply.github.com>

.changeset/pink-spoons-hope.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-search-backend': patch
+---
+
+Add support for filtering search results according to visibility.