Implement jwks endpoint and add test for it.

This also let to some improvements/cleanups of the other tests and fixtures.

IdentityShroud.Core/Security/RsaHelper.cs

@@ -4,4 +4,13 @@ namespace IdentityShroud.Core.Security;
 
 public static class RsaHelper
 {
+    /// <summary>
+    /// Load RSA private key from PKCS#8 format
+    /// </summary>
+    public static RSA LoadFromPkcs8(byte[] pkcs8Key)
+    {
+        var rsa = RSA.Create();
+        rsa.ImportPkcs8PrivateKey(pkcs8Key, out _);
+        return rsa;
+    }
 }