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
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