Refactoring Service classes

Introduced ServiceHelper class and ApiDefintion class to provide helper
methods for submitting  API requests.
1. ApiDefinition class provides the means to specify the API request parameters using typical for Mambu request types (get entity details, get list of entities, post an operation for an entity, etc.)
2. ServiceHelper class uses provided ApiDefinition to build the url, execute request and parse the response.
3. Service classes (e.g., LoansServices, ClientService, etc.) can now specify the ApiDefinitions for their APIs and use ServiceHelper to execute the request instead of building each request from scratch.

**  For now only LoansService and ClientService were modified to demonstrate this approach. Once everything is confirmed and finalized - other Services can be updated accordingly.

Other changes:
ClientService: removed obsolete, non json methods to create basic client with just first/last name and some personal details. New Json create can be used instead.
Enum values should be uppercase -> GET_ENTITY