#2257 Add Work Item Saved Views API support

This adds a WorkItemSavedViewsService for GitLab's Work Item Saved Views GraphQL API, covering issue #2257.

It supports getting, listing, creating, updating, and deleting saved views, as well as subscribing and unsubscribing.

Filters is a fully typed WorkItemSavedViewFilters struct (~35 fields, plus nested Not/Or/HierarchyFilters/Status/CustomField sub-filters), verified field-by-field against GitLab's live GraphQL schema. DisplaySettings stays json.RawMessage since the schema declares it as a genuinely untyped JSON scalar.

The implementation follows the same patterns used in the existing Work Items service (ListWorkItemsOptions' field-naming conventions in particular). Tests check the full GraphQL request, including the query and all variables, for every method - 47 tests total.

This is marked experimental since the underlying GitLab API is still a work in progress and may introduce breaking changes.

Note for anyone tracking earlier revisions: CreateSavedViewOptions/UpdateSavedViewOptions were renamed to CreateWorkItemSavedViewOptions/UpdateWorkItemSavedViewOptions to match this package's naming convention.