Moved `PluginsManager` and `PluginsManagerType` from `servers::plugin::manager` to the `servers::plugin` module.
Added some comments in code.
Implemented function `into()` to the `PluginsManager` struct.
Now function `on_load` will be executed if the plugin is loads.
Added span to the logger on TRACE level in plugin loader.
Fixed clippy warning from previous commit.
Changed map type in Client struct to Arc<Mutex<HashMap<String, ClientMapValue>>>.
Implemented functions insert_key, get_value and delete_key to the Client type.
Re-export servers::server::ClientMapValue in servers::plugins::prelude.