[api] add billingtype consts
[api] add billingtype consts

ASKozienko/Payum,Payum/Payum,formapro-forks/Payum,formapro-forks/Payum,StoreFactory/Payum,staabm/Payum,Payum/Payum,fullpipe/Payum,digisec/Payum,fullpipe/Payum,aitboudad/Payum,StoreFactory/Payum,StoreFactory/Payum,emmadery/hello-world,digisec/Payum