Add capability for mutual tls authentication

[bdurrer]

We need mutual TLS authentication for a project so I thought I give it a try.
Since it requires the SslStream to have some missing options, this wasn't possible to achieve with an extension.

[Copilot]

Pull Request Overview

This PR adds mutual TLS authentication capability to EmbedIO, allowing servers to validate client certificates during the SSL/TLS handshake. This was previously impossible to implement via extensions due to missing SslStream configuration options.

Key Changes:

• Added ClientCertificateValidationCallback property to WebServerOptions and HttpListener for configuring client certificate validation
• Modified SSL stream initialization to request and validate client certificates when a validation callback is configured
• Added comprehensive test coverage for mutual TLS scenarios including certificate acceptance, anonymous connections, and certificate rejection

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test/EmbedIO.Tests/HttpsTest.cs	Added three test cases covering mutual TLS scenarios: accepting known certificates, allowing anonymous connections, and rejecting unknown certificates
test/EmbedIO.Tests/EmbedIO.Tests.csproj	Added content include for SSL certificate files used in tests
src/EmbedIO/WebServerOptionsExtensions.cs	Added extension method WithClientCertificateValidation for fluent configuration of client certificate validation
src/EmbedIO/WebServerOptions.cs	Added ClientCertificateValidationCallback property to store the validation callback
src/EmbedIO/WebServer.cs	Updated HttpListener instantiation to pass the client certificate validation callback
src/EmbedIO/Net/Internal/HttpListenerRequest.cs	Modified IsAuthenticated property to check for mutual authentication via SslStream
src/EmbedIO/Net/Internal/HttpConnection.cs	Updated SSL stream initialization to enable client certificate requests and validation
src/EmbedIO/Net/HttpListener.cs	Added constructor parameter and internal property for client certificate validation callback

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}