codeintel: Add support for diagnostics

Created by: efritz

This adds resolvers for LSIF-powered diagnostic results and closes https://github.com/sourcegraph/sourcegraph/issues/10838.

This should be good enough to start code insights, but there are some efficiency notes I've left in the implementation for things we can do to improve the situation once we hit a large enough sample that it matters.

I also plan on adding an after parameter so that we can do legitimate paging instead of just expanding the first parameter 2x on request.