Refactor server certificate classes (breaking change).

Source/MQTTnet/Server/IMqttServerCertificateCredentials.cs

@@ -0,0 +1,4 @@
+public interface IMqttServerCertificateCredentials
+{
+    string Password { get; }
+}

Source/MQTTnet/Server/IMqttServerCredentials.cs

@@ -1,6 +0,0 @@
-using System;
-
-public interface IMqttServerCredentials
-{
-    String Password { get; }
-}

Source/MQTTnet/Server/MqttServerCertificateCredentials.cs

@@ -0,0 +1,7 @@
+namespace MQTTnet.Server
+{
+    public class MqttServerCertificateCredentials : IMqttServerCertificateCredentials
+    {
+        public string Password { get; set; }
+    }
+}

Source/MQTTnet/Server/MqttServerOptionsBuilder.cs

@@ -82,7 +82,7 @@ namespace MQTTnet.Server
             return this;
         }
 
-        public MqttServerOptionsBuilder WithEncryptionCertificate(byte[] value, IMqttServerCredentials credentials = null)
+        public MqttServerOptionsBuilder WithEncryptionCertificate(byte[] value, IMqttServerCertificateCredentials credentials = null)
         {
             _options.TlsEndpointOptions.Certificate = value;
             _options.TlsEndpointOptions.CertificateCredentials = credentials;

Source/MQTTnet/Server/MqttServerTlsTcpEndpointOptions.cs

@@ -12,7 +12,7 @@ namespace MQTTnet.Server
 
         public byte[] Certificate { get; set; }
 
-        public IMqttServerCredentials CertificateCredentials { get; set; }
+        public IMqttServerCertificateCredentials CertificateCredentials { get; set; }
 
         public bool ClientCertificateRequired { get; set; }