Reworked encryption to use less heap allocated buffers for secrets.

Also some work on plugin system.

IdentityShroud.Core/Security/Jwt/IJwtSigner.cs

@@ -0,0 +1,20 @@
+using System.Text.Json;
+using IdentityShroud.Core.Model;
+
+namespace IdentityShroud.Core;
+
+public interface IJwtSigner
+{
+    /*
+        Of the signature and MAC algorithms specified in JSON Web Algorithms
+        [JWA], only HMAC SHA-256 ("HS256") and "none" MUST be implemented by
+        conforming JWT implementations.  It is RECOMMENDED that
+        implementations also support RSASSA-PKCS1-v1_5 with the SHA-256 hash
+        algorithm ("RS256") and ECDSA using the P-256 curve and the SHA-256
+        hash algorithm ("ES256").  Support for other algorithms and key sizes
+        is OPTIONAL.
+     */
+    IReadOnlyList<JwtSigAlgName> Algorithms { get; }    
+
+    byte[] CalculateSignature(JwtSigAlgName algName, DecryptedSigningKey key, ReadOnlySpan<byte> jwt);
+}