search: filter sub-repo perms in run.Aggregator

From my understanding of the search API code:

• Streaming search is handled by streamHandler, which uses graphqlbackend.NewSearchImplementer => searchResolver. The search implementer providers a resovler via Results() that is used to get results for streaming.
• It appears SearchResultsResolver from the searchResolver is also used in the search GraphQL API
• From there, results come from resultsBatch or resultsStreaming, both of which use resultsRecursive, which seems to handle doing things to the query until we get a newResult from evaluate()
• Within evaluate(), the code branches off into a bunch of different things again

From this understanding:

• it seems that placing the sub-repo filtering in resultsRecursive is the right thing to do, so I've gone ahead and drafted what this might look like here. Another idea could be placing it in searchResolver.Results().
  • update: https://github.com/sourcegraph/sourcegraph/pull/27189#discussion_r746786381 went with an Aggregator filter
• the actor will likely be available here because this seems to all live in the frontend.

We don't have docs yet, but some details about SubRepoPermsClient:

• It caches a site config value, off by default, that makes all permissions checks return with a Read immediately
• Sub-repo perms rules will be cached in SubRepoPermsClient (https://github.com/sourcegraph/sourcegraph/issues/26663)