Dynamic mqtt port for unit tests

[erwinkramer]

This PR assigns a dynamic port number during mqtt server start and also moves from NewtonsoftJson formatter to SystemTextJson in the mqtt tests.

Should solve port conflicts like https://github.com/cloudevents/sdk-csharp/actions/runs/24733076929/job/72352560560?pr=345#step:4:102. I assume because the .net8 and .net10 tests currently have a race condition to both claim port 52355.

Related to #192.

I verified by starting 2 mqtt servers async, with similar MqttServerOptions setup, that seems to handle it fine:

var options = new MqttServerOptionsBuilder()
    .WithConnectionBacklog(100)
    .WithDefaultEndpointPort(0)
    .WithDefaultEndpoint().Build();

var options2 = new MqttServerOptionsBuilder()
   .WithConnectionBacklog(100)
   .WithDefaultEndpointPort(0)
   .WithDefaultEndpoint().Build();

this.mqttServer = new MqttFactory().CreateMqttServer(options);
this.mqttServer2 = new MqttFactory().CreateMqttServer(options2);
var one = mqttServer.StartAsync();
var two = mqttServer2.StartAsync();

one.GetAwaiter().GetResult();
two.GetAwaiter().GetResult(); // both start fine

Console.WriteLine(options2.DefaultEndpointOptions.Port); 
Console.WriteLine(options.DefaultEndpointOptions.Port); // port differs from server 2