Implement Green Button Connect (ESPI) OAuth flow and download interval energy data

Steps

1. Register your application as a third-party (TP) with the utility's Green Button Connect My Data program to receive a client_id and client_secret; each utility runs its own authorization server.
2. Redirect the customer to the utility's authorization endpoint with response_type=code, client_id, redirect_uri, and scope (typically covering UsagePoint and related ESPI resources).
3. Exchange the returned authorization code for an access token and refresh token at the utility's token endpoint using a POST with grant_type=authorization_code.
4. Retrieve the list of usage points via GET /espi/1_1/resource/Subscription/<SUBSCRIPTION_ID>/UsagePoint using the access token as a Bearer token in the Authorization header.
5. Download interval block data via GET on the ElectricPowerUsageSummary or IntervalBlock resource paths; parse the Atom/XML feed, extracting IntervalReading values, their start timestamps (epoch seconds), and duration.