Add abstraction layer for server certificate.

Source/MQTTnet/Certificates/BlobCertificateProvider.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Security.Cryptography.X509Certificates;
+
+namespace MQTTnet.Certificates
+{
+    public class BlobCertificateProvider : ICertificateProvider
+    {
+        public BlobCertificateProvider(byte[] blob)
+        {
+            Blob = blob ?? throw new ArgumentNullException(nameof(blob));
+        }
+
+        public byte[] Blob { get; }
+
+        public string Password { get; set; }
+
+        public X509Certificate2 GetCertificate()
+        {
+            if (string.IsNullOrEmpty(Password))
+            {
+                // Use a different overload when no password is specified. Otherwise the constructor will fail.
+                return new X509Certificate2(Blob);
+            }
+
+            return new X509Certificate2(Blob, Password);
+        }
+    }
+}

Source/MQTTnet/Certificates/ICertificateProvider.cs

@@ -0,0 +1,9 @@
+using System.Security.Cryptography.X509Certificates;
+
+namespace MQTTnet.Certificates
+{
+    public interface ICertificateProvider
+    {
+        X509Certificate2 GetCertificate();
+    }
+}

Source/MQTTnet/Certificates/X509CertificateProvider.cs

@@ -0,0 +1,22 @@
+#if !WINDOWS_UWP
+using System;
+using System.Security.Cryptography.X509Certificates;
+
+namespace MQTTnet.Certificates
+{
+    public class X509CertificateProvider : ICertificateProvider
+    {
+        readonly X509Certificate2 _certificate;
+
+        public X509CertificateProvider(X509Certificate2 certificate)
+        {
+            _certificate = certificate ?? throw new ArgumentNullException(nameof(certificate));
+        }
+
+        public X509Certificate2 GetCertificate()
+        {
+            return _certificate;
+        }
+    }
+}
+#endif