This is based on the API::Helpers::Caching#present_cached method
that we use for some Grape API endpoints.

Since that is specific for Grape API endpoints, we need something
that can work on Rails controllers. This exposes a new
`render_cached` helper when `Gitlab::Caching::Helpers` module is
included in a Rails controller.

Currently uses the same method signature but the `render_cached`
method calls `render` instead of just calling `body` with the
precompiled json. This is to ensure that the behavior when `render`
is called is kept.

Tested this in a PoC merge request
(https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63644) and
it shows promising results for
`Projects::MergeRequestsController#discussions` as it dropped the
response time from ~9s to ~1.5s in a large MR scenario
(100+ discussions).