insights: Add endpoint for RelatedInsightsForRepo

Created by: CristinaBirkel

Part of https://github.com/sourcegraph/sourcegraph/issues/38428 Builds on: https://github.com/sourcegraph/sourcegraph/issues/39121

This endpoint will get insights with a select:repo field that match the file passed into the endpoint. See the main issue for more details. This can just return an array of matching insight ids.