Add x-gitlab-tier OpenAPI annotation for endpoint tiers

Alberto Bignottirequested to merge
593084-x-gitlab-tier into master

What does this MR do and why?

EE REST endpoints have no machine-readable way to express which paid subscription tier they require, so consumers of the OpenAPI spec cannot tell a Premium endpoint from an Ultimate one. This registers :tier in the gitlab-grape-openapi annotations config, so route_setting :tier, :premium (or :ultimate) on any EE Grape endpoint emits x-gitlab-tier in the generated OpenAPI v3 spec — reusing the generic annotation pipeline already behind x-gitlab-lifecycle.

The config change is a single mapping entry; the gem's converter and model already handle arbitrary annotation keys, so no gem change is needed. Two endpoints are seeded as working examples: the group epics list (premium) and the vulnerability GET (ultimate) — both tiers are correct for those features. CE endpoints are implicitly Free and carry no annotation.

The API/RouteSettingTier RuboCop cop (already in the tree) restricts values to :premium/:ultimate, so invalid tiers fail lint.

References

Resolves: #593084 (closed) Part of: gitlab-org/quality&366

Screenshots or screen recordings

How to set up and validate locally

Regenerate the spec and confirm the annotation appears only on the seeded EE endpoints:

bundle exec rake gitlab:openapi:v3:generate
grep -n "x-gitlab-tier" doc/api/openapi/openapi_v3.yaml

Expected — exactly three lines (one Ultimate; two Premium for the epics path aliases), and none on CE endpoints:

4589:      x-gitlab-tier: ultimate   # getApiV4VulnerabilitiesId
9166:      x-gitlab-tier: premium    # /api/v4/groups/{id}/epics
9398:      x-gitlab-tier: premium    # /api/v4/groups/{id}/-/epics

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