Create a background worker to trigger npm metadata generation

🌱 Context

Since a metadata generation should happen in the background we need to have a background worker for that.

⚒ Proposal

Create a background worker that triggers Packages::Npm::MetadataCacheService

Important

The worker needs to be an idempotent: https://docs.gitlab.com/ee/development/sidekiq/idempotent_jobs.html#deduplication