Remove standalone server dependency on Unity

[Extremelyd1]

Fixes #70.

Server-side code depended on Unity due to the packet handlers using a type from Unity, because client-side dispatches handlers to Unity's main thread.

This PR fixes the issue by introducing an interface for dispatchers, where a server-side implementation dispatches immediately, while the client-side implementation dispatches by running it on main thread of Unity. Due to classes implementating an interface, the Unity types are no longer loaded when creating the packet handler registry instances on the server-side.

[Liparakis]

One small thing i would change -- i would convert the interfaces to Singleton and save resources on the Dispatcher allocations.

/// <summary>
/// Implementation of packet handler dispatcher to immediately invoke the handler directly for the server-side.
/// </summary>
internal class ServerPacketHandlerRegistryDispatcher : IPacketHandlerRegistryDispatcher {
    public static readonly ServerPacketHandlerRegistryDispatcher Instance = new();
    private ServerPacketHandlerRegistryDispatcher() { }
    public void Dispatch(Action action) => action.Invoke();
}

/// <summary>
/// Implementation of packet handler dispatcher to invoke the handler on Unity's main thread.
/// </summary>
internal class ClientPacketHandlerRegistryDispatcher : IPacketHandlerRegistryDispatcher {
    public static readonly ClientPacketHandlerRegistryDispatcher Instance = new();
    private ClientPacketHandlerRegistryDispatcher() { }
    public void Dispatch(Action action) => ThreadUtil.RunActionOnMainThread(action);
}

//PacketManager.cs
private readonly PacketHandlerRegistry<ClientExamplePacketId, ClientPacketHandler> _clientExampleRegistry = new(
        "Example", ClientPacketHandlerRegistryDispatcher.Instance
 );
private readonly PacketHandlerRegistry<ServerExamplePacketId, ServerPacketHandler> _serverExampleRegistry = new(
        "Example", ServerPacketHandlerRegistryDispatcher.Instance
);