search-backend-node: track document visibility permission by type in IndexBuilder

Tracking permissions by collator ensures that all the documents of a
given type are authorized using the same permission. This is not
critical for result-by-result authorization, but will likely be a
useful constraint when adding in-engine permission filtering to
specific search engines in the future.

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

.changeset/angry-carrots-brush.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-search-backend-node': patch
+---
+
+Track visibility permissions by document type in IndexBuilder