feat(workitems): add experimental WorkItemsRESTService with GetWorkItem

What does this MR do?

Adds WorkItemsRESTService, a separate service for the work items REST API, starting with one method:

wi, _, err := client.WorkItemsREST.GetWorkItem("my-group/my-project", 41, &gitlab.GetWorkItemRESTOptions{
	Fields:   []gitlab.WorkItemFieldValue{gitlab.WorkItemFieldState, gitlab.WorkItemFieldAuthor},
	Features: []gitlab.WorkItemFeatureValue{gitlab.WorkItemFeatureDescription, gitlab.WorkItemFeatureLabels},
})

GetWorkItem calls GET /namespaces/:id/-/work_items/:work_item_iid, so it works for both project and group work items. It accepts the API's fields and features query parameters.

The GraphQL-based WorkItemsService is unchanged. Callers choose which API to use.

Why

This follows the second option in !3064 (comment 3964397033): a new service rather than switching WorkItemsService to REST, so existing users of the GraphQL methods aren't affected and can move to REST when they want. Naming follows the client-go convention: the struct is WorkItemsRESTService, the interface WorkItemsRESTServiceInterface, and the client field WorkItemsREST.

The first consumer is glab's hidden work-items-experimental get command: gitlab-org/cli!4010 (for gitlab-org/cli#8585)

Notes for reviewers

• Experimental: the REST API is behind the work_item_rest_api feature flag (off by default) and returns 404 while it's off. The service carries the same experimental note as WorkItemsService.
• API docs: the REST API docs page (doc/api/work_items.md, to be published at https://docs.gitlab.com/api/work_items/) is being written and isn't published yet. The godoc links point to its planned location.
• Plain values unless the zero value is valid: the API returns only id, iid, global_id and title unless other fields are requested. Fields whose zero value can't be real data (State, WebURL, …) are plain values. Fields where false or 0 is a valid answer (Confidential, UserDiscussionsCount, …) are pointers, so nil means not requested. Fields the API can leave out are tagged omitzero, so encoded JSON contains only what the API returned.
• Features: only the features glab needs first are typed (assignees, description, hierarchy, labels, milestone, start_and_due_date). The others follow in later MRs.
• Test fixture is a real response captured from a local GDK, with host and paths replaced.

How to test

go test -race -run TestWorkItemsREST .