[FEATURE] Add select component

[albx]

Implement a BitSelectField component following the design and UX patterns of the Bootstrap Italia Select component.

Requirements:

• Create a reusable BitSelectField component inspired by Bootstrap Italia's Select (docs).
• Support for single and multiple selection modes.
• Expose properties for items (options), option grouping, placeholders, and disabled/read-only states.
• Support for custom labels, hints, and validation messages.
• Allow customizable styling and templates for options.
• Provide keyboard navigation and full accessibility (ARIA attributes, screen reader support).
• Ensure responsive design and mobile-friendly interaction.
• Support validation and display of error/success messages as per Bootstrap Italia standards.

Acceptance Criteria:

• BitSelectField visually and functionally matches the Bootstrap Italia Select component as closely as possible within Blazor constraints.
• Component supports both single and multi-select scenarios, including model binding.
• Properties for items, labels, help text, placeholders, disabled/read-only, and validation are functional and documented.
• Keyboard navigation and accessibility requirements are fully met (including ARIA attributes).
• Responsive behavior is present and properly documented.
• Validation and feedback messages are displayed according to Bootstrap Italia guidelines.
• Example usage and documentation is provided in the repo and demo site.

References

• Bootstrap Italia Select: https://italia.github.io/bootstrap-italia/docs/form/select/