Add Support for UserFields in ZendeskApi_v2

[ruannbotes]

Summary: This pull request adds support for managing UserFields within the ZendeskApi_v2 package. It includes both synchronous and asynchronous methods for CRUD operations on UserFields. The implementation aligns with the existing structure for managing TicketFields, ensuring consistency and ease of use.

Changes Include:

API Methods: Implemented sync and async methods for UserFields operations. Models: Added UserField, GroupUserFieldResponse, and IndividualUserFieldResponse models to represent UserFields and responses. Constants: Introduced UserFieldTypes constants for standardized handling of field types. Requests Update: Extended Users requests to support the new UserFields functionality. Unit Tests: Updated UsersTests to cover the new UserField methods, including both sync and async operations. Testing:

Added comprehensive unit tests to ensure proper functionality for all new methods. Tests include validation of input and output for the UserFields operations, ensuring they work as expected.