don't obtain a Client from storage on each poll

First verify if the client is authenticated.
Then the state of the device authorization.
If all is good, we take the Client from Storage.

pkg/oidc/device_authorization.go

@@ -26,5 +26,4 @@ type DeviceAuthorizationResponse struct {
 type DeviceAccessTokenRequest struct {
 	GrantType  string `json:"grant_type"`
 	DeviceCode string `json:"device_code"`
-	ClientID   string `json:"client_id"`
 }