search: Enable sub-repo filtering

Created by: ryanslade

This change adds sub-repo filtering in the search aggregator.

It is very similar to the previous PR submitted by @bobheadxi, https://github.com/sourcegraph/sourcegraph/pull/27189, as after a lot of back and forth we decided that given there was a single place we could add a filter, this was the most non-intrusive way of enabling it in the search path.

I've tested it by manually setting some filtering rules and we plan to add end to end coverage later.

Closes https://github.com/sourcegraph/sourcegraph/issues/28242