fix: use AuthURLParam to set client id and secret

Co-authored-by: 3v1n0 <marco.trevisan@canonical.com>

workaround to deal with golang/oauth2#320

tldr is that IDP servers tend to not be fully compliant with how client
credentials are passed and have bespoke arrangements so anything goes
this enforces the standard implementation from the RFC and has it working
for any RFC compliant OIDC server

full info here golang/oauth2#320

internal/broker/broker.go

@@ -357,7 +357,20 @@ func (b *Broker) generateUILayout(session *sessionInfo, authModeID string) (map[
 	case authmodes.Device, authmodes.DeviceQr:
 		ctx, cancel := context.WithTimeout(context.Background(), maxRequestDuration)
 		defer cancel()
-		response, err := session.authCfg.oauth.DeviceAuth(ctx)
+
+		var authOpts []oauth2.AuthCodeOption
+
+		// workaround to cater for fully RFC compliant oauth2 server which require this
+		// extra option, public providers tend to have bespoke implementation for passing client
+		// credentials that completely bypass this
+		// full explanation in https://github.com/golang/oauth2/issues/320
+		if secret := session.authCfg.oauth.ClientSecret; secret != "" {
+			// TODO @shipperizer verificationMethod should be a configurable value
+			verificationMethod := "client_post"
+			authOpts = append(authOpts, oauth2.SetAuthURLParam(verificationMethod, secret))
+		}
+
+		response, err := session.authCfg.oauth.DeviceAuth(ctx, authOpts...)
 		if err != nil {
 			return nil, fmt.Errorf("could not generate Device Authentication code layout: %v", err)
 		}