Add reactive caching worker for external services

What does this MR do?

Introduces a separated worker to handle reactive caching that performs external requests.

This worker needs to stay in lower priority given it's not possible to predict how external services will perform. I.e. this work should not block other work with higher priority (see https://docs.gitlab.com/ee/development/sidekiq_style_guide.html#jobs-with-external-dependencies).

The changes here doesn't affect anything yet, as the new worker is still not used (mostly a groundwork that will span into multiple merge requests.

Part of gitlab-com/gl-infra/scalability#224 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content