Add eventCountsByUserId query to GraphQL API

[msk]

We need to implement a new query eventCountsByUserId in our GraphQL API to return EventCounts containing the most frequent user IDs in the events.

Background

• This feature was requested by @sehkone
• Currently, we don't have user ID information in the database
• Database work to store user IDs needs to be completed before this query can be implemented

Implementation Details

• The new query should be named eventCountsByUserId
• It should return EventCounts with the most frequent user IDs
• For implementation guidance, refer to existing graphql::event::event_counts_by_* functions

Additional Notes

• This issue was previously tracked in the @petabi organization, but is now being handled in the aicers/review-web repository as it's responsible for the GraphQL API