Split order_most_active_desc scope in EE::Ci::Runner to make it possible to compute most active runners based on namespace ID

Currently, the order_most_active_desc scope is doing too much. It is joining with ci_running_builds and grouping by :id/sorting by most used. In order to allow finding the top active group runners in a namespace, it would be better to split the current scope in 2. Something like:

        scope :with_top_running_builds, -> do
          joins(
            <<~SQL
            INNER JOIN (
              SELECT "runner_id", ROW_NUMBER() OVER(PARTITION BY "runner_id" ORDER BY "runner_id") as rn
              FROM "ci_running_builds"
            ) as "limited_builds" ON "limited_builds"."runner_id" = "ci_runners"."id"
                                  AND "limited_builds".rn <= #{MOST_ACTIVE_RUNNERS_BUILDS_LIMIT}
            SQL
          )
        end

        scope :with_top_running_builds_by_namespace_id, ->(namespace_id) do
          joins(
            <<~SQL
            INNER JOIN (
              SELECT "runner_id", ROW_NUMBER() OVER(PARTITION BY "runner_id" ORDER BY "runner_id") as rn
              FROM "ci_running_builds"
              WHERE "ci_running_builds"."namespace_id" = #{namespace_id}
            ) as "limited_builds" ON "limited_builds"."runner_id" = "ci_runners"."id"
                                  AND "limited_builds".rn <= #{MOST_ACTIVE_RUNNERS_BUILDS_LIMIT}
            SQL
          )
        end

        scope :order_most_active_desc, -> do
          group(:id).reorder('COUNT(limited_builds.runner_id) DESC NULLS LAST', arel_table['id'].desc)
        end

This would allow writing the following query:

Ci::Runner.group_type.with_top_running_builds_by_namespace_id(namespace.id).order_most_active_desc.limit(5)

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