Build an OpenADR 3.0 VEN client that subscribes to programs, polls for events, and submits opt responses via the REST API

Verified steps

1. Register a client credential with the VTN's OAuth 2.0 authorization server (client_credentials grant); OpenADR 3.0 uses a RESTful API defined in the openadr3.yaml OpenAPI spec — all payloads are JSON, replacing the SOAP/XML of 2.0b.
2. Call GET /programs to discover available demand response programs; parse the array of Program objects and store programID values for the programs your VEN participates in.
3. Poll GET /events?programID={id} on a configurable interval (check VTN documentation for recommended polling frequency); process each Event object, which includes eventName, intervals (an array with dtstart, duration, and payloads), and priorities.
4. For each active event, evaluate the payload against your DER's operating constraints; submit a POST /reports body with an oadrReport containing your baseline and actual consumption or curtailment telemetry if the program requires reporting.
5. Submit an opt response via POST /subscriptions or by posting to the VTN's opt endpoint with an Opt object containing optType ('optIn' or 'optOut'), programID, eventID, and venID.
6. Implement exponential backoff with jitter on polling; on HTTP 429 responses honor the Retry-After header if present, and log events with their intervals to a local store so that a temporary poll failure does not cause missed event processing.