[FEATURE] Pagination - Add Changer section (records per page dropdown)

[albx]

Add support for the Changer section of the Bootstrap Italia pagination component.

The Changer feature allows users to select from a dropdown the number of records to display per page.

⚠️ Blocked by #54 — This feature requires the Dropdown component, which will not be implemented. This issue is tracked for completeness but cannot be implemented until that dependency is resolved.

Requirements:

• Add a ShowChanger (or equivalent) parameter to BitPagination to enable the Changer mode
• Render a dropdown (using BitDropdown once available) allowing the user to pick the number of items per page
• Expose a PageSizeOptions parameter (e.g. IEnumerable<int>) for configuring available page size values
• Expose a PageSize parameter and a PageSizeChanged callback for two-way binding
• Ensure the dropdown is properly labelled for screen readers

Acceptance Criteria:

• Add bUnit tests covering the Changer rendering and interaction
• Add documentation / story for the Changer variant
• Behaviour matches the Bootstrap Italia reference: https://italia.github.io/bootstrap-italia/docs/componenti/paginazione/#changer

References

• Bootstrap Italia – Paginazione / Changer: https://italia.github.io/bootstrap-italia/docs/componenti/paginazione/#changer
• Blocked by [FEATURE] Implement BitDropdown component #54 (Dropdown component)