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/OpenSslGostKeyOptions.cs

@@ -0,0 +1,29 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace StellaOps.Cryptography.Plugin.OpenSslGost;
+
+public sealed class OpenSslGostKeyOptions
+{
+    [Required]
+    public string KeyId { get; set; } = string.Empty;
+
+    [Required]
+    public string Algorithm { get; set; } = SignatureAlgorithms.GostR3410_2012_256;
+
+    [Required]
+    public string PrivateKeyPath { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Optional environment variable containing the passphrase for the private key PEM (avoids inline secrets).
+    /// </summary>
+    public string? PrivateKeyPassphraseEnvVar { get; set; }
+
+    /// <summary>
+    /// Optional certificate (PEM/DER/PFX) to populate JWK x5c entries.
+    /// </summary>
+    public string? CertificatePath { get; set; }
+
+    public string? CertificatePasswordEnvVar { get; set; }
+
+    public GostSignatureFormat SignatureFormat { get; set; } = GostSignatureFormat.Der;
+}