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);
+}

IdentityShroud.SecretProviders/IdentityShroud.SecretProviders.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0</TargetFramework>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+    </PropertyGroup>
+
+    <ItemGroup>
+      <ProjectReference Include="..\IdentityShroud.PluginSupport\IdentityShroud.PluginSupport.csproj" />
+    </ItemGroup>
+
+</Project>

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);
+    }
+}