[Subsystem] Implement Plugin Subsystem Architecture (Port of Ivy.Plugin.Abstractions)

Problem Statement

The original C# Ivy-Framework features Ivy.Plugin.Abstractions and a dynamic plugins/ directory system for third-party extensions, custom widget registrations, and backend message handlers.

Rusty-Framework currently lacks a plugin system. Developers cannot package or load third-party extension modules dynamically into a rusty-server application.

Missing Functionality

• Plugin and PluginModule trait definitions in Rust.
• Dynamic widget and custom WebSocket message handler registration mechanisms.
• Plugin lifecycle hooks (initialize, register_widgets, register_handlers, shutdown).
• Schema validation for plugin configuration.

Proposed Architecture & Design

1. Create a rusty-plugin module (or crate) defining Plugin, PluginContext, and PluginRegistry.
2. Add plugin registration methods to rusty-server::ServerBuilder.
3. Support plugin-scoped state injection and middleware registration.
4. Implement an example plugin crate (rusty-plugin-example) demonstrating custom widget and route registration.

Acceptance Criteria

• Plugin trait defined with lifecycle hooks (initialize, register_widgets, shutdown).
• rusty-server supports loading and registering plugins during startup.
• Custom widgets and message handlers registered by plugins render and execute cleanly.
• Example plugin implementation provided with unit and integration tests.

Start by reviewing the proposed rusty-plugin module or crate, rusty-server::ServerBuilder, and the stated lifecycle hooks and registration mechanisms. Compare the original C# Ivy.Plugin.Abstractions behavior with the requested Rust architecture. Done means plugins can load during startup, register widgets and message handlers, support configuration validation, and include the example plugin with unit and integration tests.