add onServerPage function

[dcanob]

This pull request adds support for server-side pagination to the VirtualSelect component, allowing options to be loaded in pages from the server as the user scrolls through the dropdown. The update includes new properties, methods, and documentation to help developers implement and use this feature efficiently.

Server-side pagination support:

• Added a new onServerPage callback property and serverPageSize option to enable server-side pagination, allowing options to be loaded in pages from the server. The component now tracks pagination state (currentPage, hasMorePages, etc.) and automatically loads the next page when the user scrolls near the bottom of the dropdown. [1] [2] [3] [4] [5] [6] [7] [8]
• Introduced the setServerPaginatedOptions method, which appends new options from the server and updates pagination state. This method is documented and demonstrated in the usage examples. [1] [2] [3]
• Enhanced search integration with server-side pagination, ensuring that a new search resets pagination and loads the first page of filtered results. [1] [2]
• Updated the component to automatically load the first page of options when the dropdown is opened, and to clear the search value when the dropdown is closed. [1] [2]

Documentation updates:

• Added detailed documentation and usage examples for server-side pagination in docs/examples.md, docs/methods.md, and docs/properties.md, including property descriptions, code samples, and notes on expected behavior. [1] [2] [3] [4]

These changes make it much easier to work with large datasets by loading only the necessary options as needed, improving both performance and user experience.