Reworked encryption to use less heap allocated buffers for secrets.

Also some work on plugin system.

IdentityShroud.SecretProviders/PlainSecret.cs

@@ -0,0 +1,18 @@
+using System.Security.Cryptography;
+
+namespace IdentityShroud.SecretProviders;
+
+public sealed class PlainSecret(byte[] secret) : IDisposable
+{
+    private bool _disposed;
+
+    public ReadOnlySpan<byte> Secret => secret;
+
+    public void Dispose()
+    {
+        if (_disposed) 
+            return;
+        _disposed = true;
+        CryptographicOperations.ZeroMemory(secret);
+    }
+}