feat: Preloading User Experience SLIs' feature categories JSON schema

What does this MR do and why?

This MR re-applies a previous change, reverted here, adding a way to preload the feature categories' JSON schema.

Previously, the remote call broke the airgapped QA environment. By preloading the schema, it will live in the cache and not perform HTTP calls.

Also related to https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/incident-management/-/issues/2479.

The preloading functionality is being added to labkit: gitlab-org/ruby/gems/labkit-ruby!229 (merged). We should merge this as soon as possible to remediate the issue happening in the incident mentioned, and later bump the labkit version to use the preloading functionality.

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