Add eventCountsByHostname query to GraphQL API

[msk]

We need to implement a new query eventCountsByHostname in our GraphQL API to return EventCounts containing the most frequent hostnames in the events.

Background

• This feature was requested by @sehkone
• Currently, we don't have hostname information in the database
• @syncpark will be working on adding hostname information to the database

Implementation Details

• The new query should be named eventCountsByHostname
• It should return EventCounts with the most frequent hostnames
• 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