Fix incomplete verification URI issue in device auth flow (#838)

Adds functionality to support Identity Provider (IdP) managers 
that do not support a complete verification URI in the 
device authentication flow. 
In cases where the verification_uri_complete field is empty,
the user will be prompted with their user_code, 
and the verification_uri  field will be used as a fallback

client/internal/oauth.go

@@ -148,6 +148,11 @@ func (h *Hosted) RequestDeviceCode(ctx context.Context) (DeviceAuthInfo, error)
 		return DeviceAuthInfo{}, fmt.Errorf("unmarshaling response failed with error: %v", err)
 	}
 
+	// Fallback to the verification_uri if the IdP doesn't support verification_uri_complete
+	if deviceCode.VerificationURIComplete == "" {
+		deviceCode.VerificationURIComplete = deviceCode.VerificationURI
+	}
+
 	return deviceCode, err
 }