Vue App, Router, Pinia and Skeleton components for Service Accounts

What does this MR do and why?

Follows the Pinia guidlines for Gitlab:

https://docs.gitlab.com/ee/development/fe_guide/pinia.html#best-practices

Bootstraps the Vue app and router with placeholder skeleton components to allow parallel development of service accounts UI. Behind an existing feature flag.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

• Enable feature flag Feature.enable(:service_accounts_crud) in a rails console

• Confirm you see 'Service Accounts" in Top- Level Group -> Settings Nav

• Confirm you see 'Service Accounts" in Admin Settings -> Settings Nav

• Confirm page reachable https://gdk.test:3000/admin/application_settings/service_accounts/ placeholder text

• Confirm Access Token Page for a given Service Accoutn ID is reachable https://gdk.test:3000/admin/application_settings/service_accounts/<ID>/access_tokens

• Confirm page reachable https://gdk.test:3000/<GROUP>/service_accounts/ placeholder text

• Confirm Access Token Page for a given Service Account ID is reachable https://gdk.test:3000/<GROUP>/service_accounts/<ID>/access_tokens

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After (group level)
N/A

Before	After (instance/admin)
N/A

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #509866 (closed)