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

[rorychatt]

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.