Capture AWS role for creating EKS clusters

This MR is part of delivering #22392 (closed). It does not deliver the whole feature. It only delivers the functionality described below:

• A form that displays two pieces of information in read-only fields: Account ID and External ID
• The form captures a Role ARN in a text field.
• When the Role ARN is set, the "Authenticate" button is enabled.
• When the "Authenticate" button is clicked, if the authentication is successful, the form to create an AWS EKS cluster is displayed.
• If the authentication fails, a banner is displayed indicating the error.

The form looks like this:

Testing

Before testing

• Enable create_eks_clusters feature flag
• Enter service account credentials in Admin -> Settings -> Integrations -> EKS Integration

How to test

• In the sidebar menu, select Operations -> Kubernetes
• Click "Add Kubernetes cluster" button.
• Click "Amazon EKS" button.
• You should see the Role ARN form.