feat(plugin): implement load and unload functionality for plugins

[MengHanLOVE1027]

Summary

Adds runtime per-plugin load, unload and reload commands, so operators can swap
a single plugin without a full server reload. Works for both C++ and Python plugins.

Changes

• Added /load <plugin> — loads and enables a single plugin by name.
• Added /unload <plugin> — disables and unloads a single plugin.
• Added /reload <plugin> — reloads a single plugin (unload + load).
• /load with no arguments loads all plugins; /unload with no arguments unloads all.
• Plugin names resolve against the plugins/ directory (case-sensitive):
  .dll/.so files strip the endstone_ prefix, .whl files resolve the distribution name.
• Unloading a plugin that another loaded plugin hard-depends on is refused and
  reported to the sender. The force parameter is reserved in the API for future use.
• New permission nodes: endstone.command.load, endstone.command.unload.
• Refactored reload() into reusable unloadAllPlugins() / loadAllPlugins().

API

• PluginManager::unloadPlugin(const std::string &name, bool force = false)
• PluginLoader::unloadPlugin(Plugin &plugin) (default no-op)

Both are additive and non-breaking.

Motivation

Lets operators hot-swap a single plugin during development or debugging without
restarting the whole server, and complements the existing full reload.

Testing (not finish yet)

• Built successfully (clang-cl / clang++)
• Manual: /unload removes the plugin and its commands; /load restores them

• C++ plugin DLL is released on unload and can be reloaded
• Python plugin: sys.modules cleared so a reloaded plugin picks up new code
• Dependency refusal: /unload B is rejected while plugin A hard-depends on B
• reload (full reload) still works