Add support for pluggable stream provider to enable SOCKS5 proxies (#2251)

* Add SOCKS5 proxy support via pluggable stream provider

Introduces MQTTnet.Extensions.Socks5, enabling SOCKS5 proxy support for MQTTnet TCP transport. Adds IMqttClientStreamProvider interface, Socks5StreamProvider implementation, and related configuration options. Updates MqttTcpChannel to support stream providers and TLS layering. Includes unit tests with a fake SOCKS5 server and usage samples. Solution and project references updated accordingly.

* Add SOCKS5 options validation for invalid configurations

* Tighten SOCKS5 RFC compliance checks

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Co-authored-by: Copilot <Copilot@users.noreply.github.com>

Source/MQTTnet.Tests.Extensions.Socks5/MQTTnet.Tests.Extensions.Socks5.csproj

@@ -0,0 +1,28 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <OutputType>Exe</OutputType>
+        <IsPackable>false</IsPackable>
+        <LangVersion>default</LangVersion>
+        <Nullable>disable</Nullable>
+        <NoWarn>$(NoWarn);CA1707;CA1859</NoWarn>
+        <EnableMSTestRunner>true</EnableMSTestRunner>
+        <RootNamespace>MQTTnet.Tests.Extensions.Socks5</RootNamespace>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <PackageReference Include="MSTest" Version="4.0.2" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\MQTTnet\MQTTnet.csproj" />
+        <ProjectReference Include="..\MQTTnet.Server\MQTTnet.Server.csproj" />
+        <ProjectReference Include="..\MQTTnet.Extensions.Socks5\MQTTnet.Extensions.Socks5.csproj" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <None Include=".\..\..\global.json"
+                Link="global.json"
+                CopyToOutputDirectory="PreserveNewest"/>
+    </ItemGroup>
+</Project>