vendor: import KillerPDF 1.7.5 source (GPL-3.0) as the base for MMD PDF

Services/Signing/ICertificateProvider.cs

@@ -0,0 +1,21 @@
+using System.Security.Cryptography.X509Certificates;
+
+namespace KillerPDF.Services.Signing
+{
+    /// <summary>
+    /// A source of a signing certificate (with its private key). Kept as an interface so the only
+    /// Windows-only piece is the certificate-store provider; .pfx files (and, later, OS keychains)
+    /// keep the rest of the signing module portable for the planned Linux/Mac port.
+    /// </summary>
+    internal interface ICertificateProvider
+    {
+        /// <summary>Human-readable label for the UI (file name, or the cert's subject).</summary>
+        string DisplayName { get; }
+
+        /// <summary>
+        /// Returns the certificate (must have a usable private key). Throws on failure - a wrong
+        /// .pfx password, a missing private key, an unreadable file, etc.
+        /// </summary>
+        X509Certificate2 GetCertificate();
+    }
+}