insights: series status is on the wrong level

Created by: coury-clark

Currently the series status (the object that has information about the backfill progress) is connected to the time series data object https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/cmd/frontend/graphqlbackend/insights.graphql?L84.

This causes problem for dynamically generated insight series, because if the resolver is empty there is no status information to render the loading states.

Ideally this information is provided at the data series definition level, both to reduce duplication on response and to match the logical location of where this status belongs.