Use background worker to fetch credentials for EKS cluster creation form

Problem to solve

In !20020 (merged) we reduced the number of external requests performed by the EKS creation flow down to one (requesting temporary credentials that the frontend will then use to populate the form). However this external request is still performed within the context of the Rails request, which could cause issues (500, timeout) if GitLab was unable to connect to AWS.

Proposal

When a user provides their AWS role details for EKS cluster creation, use a background worker to fetch the credentials from AWS. The frontend can then fetch these details in a subsequent request.

What does success look like, and how can we measure that?

External requests are only performed from background workers at any point in the EKS cluster creation process.