Reworked encryption to use less heap allocated buffers for secrets.

Also some work on plugin system.

IdentityShroud.SecretProviders/ISecretProvider.cs

@@ -0,0 +1,15 @@
+namespace IdentityShroud.SecretProviders;
+
+/// <summary>
+/// Required interface of a SecretProvider. 
+/// </summary>
+public interface ISecretProvider
+{
+    /// <summary>
+    /// Used as a key in the registry. This same value should be used for the type field
+    /// when configuring a secret.
+    /// </summary>
+    string Key { get; }
+    
+    Task<PlainSecret> GetSecret(string configurationValue, CancellationToken ct = default);
+}