Reworked encryption to use less heap allocated buffers for secrets.

Also some work on plugin system.

IdentityShroud.PluginSupport/IPlugin.cs

@@ -0,0 +1,9 @@
+namespace IdentityShroud.PluginSupport;
+
+/// <summary>
+/// Any class that should be discovered when loading a dll should implement IPlugin
+/// </summary>
+public interface IPlugin
+{
+    
+}