Add support for ГОСТ Р 34.10 digital signatures

- Implemented the GostKeyValue class for handling public key parameters in ГОСТ Р 34.10 digital signatures.
- Created the GostSignedXml class to manage XML signatures using ГОСТ 34.10, including methods for computing and checking signatures.
- Developed the GostSignedXmlImpl class to encapsulate the signature computation logic and public key retrieval.
- Added specific key value classes for ГОСТ Р 34.10-2001, ГОСТ Р 34.10-2012/256, and ГОСТ Р 34.10-2012/512 to support different signature algorithms.
- Ensured compatibility with existing XML signature standards while integrating ГОСТ cryptography.

src/__Libraries/StellaOps.Cryptography.Plugin.OpenSslGost/OpenSslPemLoader.cs

@@ -0,0 +1,73 @@
+using System;
+using System.IO;
+using System.Security.Cryptography.X509Certificates;
+using Org.BouncyCastle.Crypto;
+using Org.BouncyCastle.Crypto.Parameters;
+using Org.BouncyCastle.OpenSsl;
+using Org.BouncyCastle.Security;
+
+namespace StellaOps.Cryptography.Plugin.OpenSslGost;
+
+internal static class OpenSslPemLoader
+{
+    public static ECPrivateKeyParameters LoadPrivateKey(string path, string? passphrase)
+    {
+        using var reader = File.OpenText(path);
+        var pemReader = string.IsNullOrEmpty(passphrase)
+            ? new PemReader(reader)
+            : new PemReader(reader, new StaticPasswordFinder(passphrase));
+
+        var pemObject = pemReader.ReadObject();
+
+        return pemObject switch
+        {
+            AsymmetricCipherKeyPair pair when pair.Private is ECPrivateKeyParameters ecPrivate => ecPrivate,
+            ECPrivateKeyParameters ecPrivate => ecPrivate,
+            _ => throw new InvalidOperationException($"Unsupported private key content in '{path}'.")
+        };
+    }
+
+    public static ECPublicKeyParameters LoadPublicKey(ECPrivateKeyParameters privateKey, X509Certificate2? certificate)
+    {
+        if (certificate is not null)
+        {
+            var bouncyCert = DotNetUtilities.FromX509Certificate(certificate);
+            var keyParam = bouncyCert.GetPublicKey();
+            if (keyParam is ECPublicKeyParameters ecPublic)
+            {
+                return ecPublic;
+            }
+        }
+
+        var q = privateKey.Parameters.G.Multiply(privateKey.D).Normalize();
+        return new ECPublicKeyParameters(q, privateKey.Parameters);
+    }
+
+    public static X509Certificate2? LoadCertificate(string? path, string? passphrase)
+    {
+        if (string.IsNullOrWhiteSpace(path))
+        {
+            return null;
+        }
+
+        if (string.Equals(Path.GetExtension(path), ".pem", StringComparison.OrdinalIgnoreCase))
+        {
+            return X509Certificate2.CreateFromPemFile(path);
+        }
+
+        var password = string.IsNullOrEmpty(passphrase) ? null : passphrase;
+        return string.IsNullOrEmpty(password)
+            ? X509CertificateLoader.LoadPkcs12FromFile(path, ReadOnlySpan<char>.Empty)
+            : X509CertificateLoader.LoadPkcs12FromFile(path, password.AsSpan());
+    }
+
+    private sealed class StaticPasswordFinder : IPasswordFinder
+    {
+        private readonly char[] password;
+
+        public StaticPasswordFinder(string passphrase)
+            => password = passphrase.ToCharArray();
+
+        public char[] GetPassword() => password;
+    }
+}