New GET KPI endpoint

Medha wants answers about how the adoption of the public Casper network is progressing. She has specific KPI’s that cannot be answered by observing the blockchain itself. These are specific product insights that need to be aggregated and watched over time.

Endpoint: **/kpis**
Method: **GET**
Description: This endpoint provides aggregated KPIs and product insights related to the adoption of the public Casper network. The response should include data points like adoption rates, user activity metrics, contract interactions, and other relevant KPIs.

Start by reviewing the existing API endpoints and how the sidecar stores or exposes historical data. Define the KPI response shape and the aggregation requirements for adoption, user activity, and contract interactions before implementation. Done means GET /kpis returns agreed, time-aware product metrics for the public Casper network.

Written by the indexing model from the issue text.