Insights: add "code host" aggregation to search aggregations / insights 4.0 feature

Created by: Joelkw

Feedback in particular from this customer. We have many large customers who've connected multiple code hosts and would get immediate value from this addition.

This helps users understand how widespread a reference is across their codebase without needing to set up a separate code insight, making search more useful to them more often and aligning with our 1-year strategy.

(So, you'd still have "repo" | "author" | "code host" as an option).

It's likely as part of this feature or a fast followup we should allow users to specify a default aggregation display (since either repo or code host are broad enough to be relevant consistently).

/cc @joelkw @felixfbecker @vovakulikov @unclejustin