Added AllowPacketFragmentationSelector option.

[xljiulang]

When I was developing the WebSocket capability of #2103, I found that the AllowPacketFragmentation of MqttServerTcpEndpointBaseOptions could not be perfectly adapted. We need to dynamically choose whether to use PacketFragmentation based on whether the IMqttChannelAdapter is a WebSocket connection, rather than simply true or false.

I also considered adding the AllowPacketFragmentationSelector option to MQTTnet.AspNetCore, but considering that MQTTnet.Server actually needs this option capability, I think it is most appropriate to add it to MqttServerTcpEndpointBaseOptions.

AllowPacketFragmentationSelector is a nullable property with a higher priority than the AllowPacketFragmentation property. It exposes the IMqttChannelAdapter to the implementer of the selector. We need to add an IsWebSocketConnection property to the IMqttChannelAdapter.

/// <summary>
///     Usually the MQTT packets can be send partially. This is done by using multiple TCP packets
///     or WebSocket frames etc. Unfortunately not all clients do support this feature and
///     will close the connection when receiving such packets. If such clients are connecting to this
///     server the flag must be set to _false_.
/// </summary>
public bool AllowPacketFragmentation { get; set; } = true;

/// <summary>
/// Select whether to AllowPacketFragmentation for an <see cref="IMqttChannelAdapter"/>.
/// Its priority is higher than the <see cref="AllowPacketFragmentation"/>.
/// </summary>
public Func<IMqttChannelAdapter, bool> AllowPacketFragmentationSelector { get; set; }

Currently I have only adapted MAllowPacketFragmentationSelector for MQTTnet.Server, because it doesn't make much sense to adapt it for the current branch of MQTTnet.AspNetCore, and it needs to be adapted after #2103.