Add add toggle to enable/disable aggregated VSA
What does this MR do and why?
This MR adds an API endpoint to enable/disable the aggregated VSA backend. The change is not user-facing. The API will be used by the frontend, see the design here: #352428 (closed)
We expect this toggle to be short-lived until the aggregated backend is fully rolled out and tested on production.
- Endpoint: PUT group/-/analytics/value_stream_analytics/use_aggregated_backend
- Payload: { enabled: true/false }
Additionally, the MR also improves the estimated_next_run_at calculation which now takes the corn job scheduling into account. We run the aggregation job every 10 minutes so we change the calculation from:
Duration between the first Aggregation record to be executed and the current Aggregation record
To:
Duration between the first Aggregation record to be executed and the current Aggregation record + number of minutes until the next scheduled aggregation.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
- 
I have evaluated the MR acceptance checklist for this MR. 
Related to #344803 (closed)