Add MqttRoutingOptions.FromAssemblies to support explicit controller assembly discovery

[Phenek]

This PR adds FromAssemblies to MqttRoutingOptions so AddMqttControllers() can use explicit assembly discovery rather than relying on default Assembly.GetEntryAssembly().

In containerized integration tests (e.g., WebApplicationFactory), GetEntryAssembly() is the test host project, which often does not include our MQTT controller types ([MqttController]). As a result, routing table is empty and MQTT messages are seen as "not consumed" even though controller methods exist.

Changes included

ServiceCollectionExtensions (MQTTnet.AspNetCore.Routing)

new method:
public static MqttRoutingOptions FromAssemblies(this MqttRoutingOptions opt, params Assembly[] assemblies) { opt.FromAssemblies = assemblies; return opt; }

Why this matters

Fixes non-deterministic route discovery in integration tests.
Matches behavior of ASP.NET MVC/Endpoint routing where explicit route assembly source is supported.
Required for test frameworks where entry assembly is not the web app object.

Example usage

services.AddMqttControllers(opt => { opt.FromAssemblies(typeof(Startup).Assembly); });