Add methods for hostIndex and customBaseUrl

if you dont specifically declare a custom base url using the set method then it uses the 1st server in the operation host index array

if no custom url is set and the operation base path array is empty however, the call throws an exception

First checks to see if a custom url is provided

If not, checks to see if operation level server is defined and uses the supplied host index (default 0)

If neither is supplied, uses the ApiClient default base path