Draft: feat(workitems): Implement CreateWorkItem().

What does this MR do and why?

Issue: #2219

This merge request adds the ability to create new work items (like tasks, issues, or epics) in GitLab projects. The changes include:

New functionality:

• Added a CreateWorkItem method that allows users to create different types of work items by providing a title and optional details like description, assignees, due dates, labels, and other metadata
• Created comprehensive input structures to handle all the different properties a work item can have when being created
• Added predefined work item type constants (Issue, Task, Epic, etc.) for easy reference

Error handling:

• Added a new error type specifically for when an item response was expected but not received
• Proper error handling for both network issues and validation errors from GitLab

Testing and mocking:

• Generated mock implementations for testing purposes
• Added comprehensive test cases covering successful creation scenarios and error cases

The implementation uses GitLab's GraphQL API to create work items and transforms user-friendly input options into the specific format required by GitLab's backend. This gives developers a clean, easy-to-use interface for programmatically creating work items while handling all the complex GraphQL formatting behind the scenes.