BE: Expose work items count by state on GraphQL (1)
Introduce a new type to expose work items counts by state on GraphQL, it could be something similar to https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/graphql/types/boards/epic_list_metadata_type.rb.
It would be nice if we could filter work items by type, so they could be re-usable for all work items views, as I am not sure if this is a requirement for work items UI we could leave this part to another iteration.
Backend portion of #382469.