Convert Grape Lifecycle directive to OpenAPI annotation

Overview

As part of gitlab-org/quality&269, we'll be defining lifecycles on REST endpoints and the methods by which to do so.

We will need to add functionality to the gitlab-grape-openapi gem to convert that information in to an OpenAPI annotation. For example (if we use the grape route_setting method to achieve this):

route_setting :api_lifecycle, :general_availability

...will convert to...

x-gitlab-api-lifecycle: "general_availability"