feat: validate plugin dependencies during plugin loading

[DevD4v3]

Summary

This PR introduces support for plugin dependencies through the new DependsOnAttribute.

During plugin loading, CPlugin.Net validates that every declared dependency is also present in the configured plugin list. If a dependency cannot be resolved, plugin loading fails with a PluginDependencyException.

This helps detect configuration errors early and prevents plugins from running with missing requirements.

Usage

[assembly: Plugin(typeof(GunGameCommand))]
// GunGame consumes IWeapon services registered by WeaponsPlugin.
// Therefore, WeaponsPlugin must also be included in the plugin configuration.
[assembly: DependsOn("WeaponsPlugin")]

namespace GunGamePlugin;

internal class GunGameCommand(IEnumerable<IWeapon> weapons) : ICommand
{
    public string Name => "gungame";
    public string Description => "Lists the available weapons.";
    public string Version => "1.0.0";

    public int Execute()
    {
        Console.WriteLine();
        Console.WriteLine("Registered weapons:");

        foreach (IWeapon weapon in weapons.OrderBy(w => w.Slot))
        {
            Console.WriteLine($"  [{weapon.Slot}] {weapon.Name} (Id: {weapon.Id})");
        }

        Console.WriteLine();
        return 0;
    }
}

If the Weapons plugin is not listed in the plugin configuration (for example, in appsettings.json or a .env file), PluginLoader.Load(...) throws:

PluginDependencyException:
The plugin 'GunGamePlugin' depends on 'WeaponsPlugin', but 'WeaponsPlugin' was not found in the plugin configuration.

Changes

• Added DependsOnAttribute.
• Added PluginDependencyException.
• Added dependency validation during plugin loading.
• Added sample plugins demonstrating plugin dependencies.
• Added XML documentation.

Motivation

Some plugins require services, commands, or other components provided by another plugin.

Previously, these relationships were implicit and configuration errors were only discovered later at runtime.

With this change, plugin dependencies become explicit and are validated during plugin loading, allowing configuration issues to be detected immediately with a clear error message.