Add view options to the Artifact Registry repositories list

What does this MR do and why?

This MR adds a "View options" toolbar control to the Artifact Registry repositories list page (EE-only). Users can toggle optional columns on and off: Type, Downloads, Size, and Last updated. The Format, Name, and Actions columns have no toggle and always show.

Column visibility persists in the browser's localStorage under the key artifact-registry-repositories-columns, so it survives a page reload.

The version list already had this kind of control, in repositories/versions/view_options.vue. This MR extracts its dropdown, Columns group, toggles, and show/hide logic into a shared component, repositories/components/view_options.vue. The version list's own file is now a thin wrapper around the shared component. Its behavior is unchanged. A new constant, REPOSITORIES_OPTIONAL_COLUMNS, defines the optional columns for the toolbar and list page to share.

This is a frontend-only, presentation-only change. It uses no feature flag. No documentation page describes the repositories list toolbar today, so this MR adds none. Jest specs cover the new shared component and extend the toolbar, table, and list page specs; the version list's own spec was trimmed to its wrapper behavior. All affected specs pass.

This implements part 2, "View options popover", of the linked work item. Parts 1 and 3 of that work item are separate.

References

• Work item: #626781
• Precedent MR (added this same control to the version list): !251881 (merged)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In a local GDK instance, open an organization's Artifact Registry repositories list at /o/<organization>/-/artifact_registry/<slug>/repositories.
2. Click the "View options" icon button in the toolbar, between the search bar and the format dropdown.
3. Toggle a column off, for example "Size". Confirm the column leaves the table and the dropdown panel stays open.
4. Reload the page. Confirm the hidden column stays hidden.
5. Open "View options" again. Confirm Format, Name, and Actions have no toggle.

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