insights: add setting to control aggregation buffer size

Created by: leonore

at the moment when we run aggregation searches we have a fixed size buffer: https://sourcegraph.sourcegraph.com/github.com/sourcegraph/sourcegraph@4b20b93342ff21a09365c931afe6756f464bf889/-/blob/enterprise/internal/insights/resolvers/aggregates_resolvers.go?L71:32-71:52 (see const)

we should change that to a setting

/cc @joelkw @felixfbecker @vovakulikov @unclejustin