refactor!: update architecture to improve extensibility

[hashemix]

📌 Summary

This PR refactors the create_server() function to accept an McpServerHandler directly, improving the SDK's flexibility and extensibility. This lays groundwork for plug-and-play session store support and other modular components.

✨ Changes Made

• create_server() now accepts a McpServerHandler directly.
• Introduced new traits to convert ServerHandler and ServerHandlerCore to a McpServerHandler.
  This enables the creation and configuration of handlers before calling create_server().

⚠ BREAKING CHANGE:
This is a minor breaking change that's easy to adopt:
when calling create_server(), simply call .to_mcp_server_handler() on your handler instance to convert it to an McpServerHandler before passing it in:

- let server =  server_runtime::create_server(server_details, transport, handler);
+ let server =  server_runtime::create_server(server_details, transport, handler.to_mcp_server_handler());

🛠️ Testing Steps

cargo make check

💡 Additional Notes

This lays groundwork for plug-and-play session store support and other modular components, in line with future support planned for HTTP frameworks like Actix and serverless environments such as AWS Lambda.