Add support for Work Items - Create
# Problem Statement
`client-go` package currently lacks support for creating Work Items.
### Proposed Solution
Create a new `WorkItemsService` with:
* **Pre-defined data structures** matching common work item fields and operations
* **High-level functions** (`CreateWorkItem()`) that abstract away GraphQL query construction
* **Automatic type conversion** from GraphQL response types to idiomatic Go types
* **Consistent API patterns** following the conventions established by `IssuesService` and other existing services
docs:
* https://docs.gitlab.com/api/graphql/reference/#mutationworkitemcreate
task