Add `ExternalCustomerService` for external (e.g. parent) customers

composer.json

.../wisc/services/cbs/external_customer/ExternalCustomer.php

...ternal_customer/ExternalCustomerInputParametersMapper.php

...ervices/cbs/external_customer/ExternalCustomerService.php

...cbs/external_customer/ExternalCustomerServiceResponse.php

...ces/cbs/external_customer/MockExternalCustomerService.php

...ces/cbs/external_customer/SoapExternalCustomerService.php

...tomer/generated/DOIT_SOA_EXTERNAL_CUSTOMER_V2_Service.php

...vices/cbs/external_customer/generated/InputParameters.php

...ices/cbs/external_customer/generated/OutputParameters.php

...sc/services/cbs/external_customer/generated/SOAHeader.php

...isc/services/cbs/external_customer/generated/autoload.php

src/main/resources/doit_soa_external_customer_v2.xml

...cbs/external_customer/MockExternalCustomerServiceTest.php

...services/cbs/external_customer/SoapExternalCustomerIT.php