Poll for a device grant

Exchanges a completed device authorization for a token. Send grant_type=urn:ietf:params:oauth:grant-type:device_code for RFC 8628 errors: HTTP 400 application/json with an error field, not a problem document. authorization_pending means keep polling at the advertised interval; slow_down means increase the polling interval by five seconds (the interval field gives the new minimum). access_denied means consent was denied or the approving session or authority is no longer valid; restart device authorization. expired_token also means restart device authorization; invalid_grant means the code is unknown or already consumed. Clients that omit grant_type use the legacy problem-shaped application/json responses: 428 for pending or slow_down, 403 for denied or invalid, and 410 for expired. Input validation failures use HTTP 400 application/problem+json.