Add EnvelopeClaimTypes for custom envelope claim strings

Introduced EnvelopeGenerator.GeneratorAPI namespace and EnvelopeClaimTypes static class. Added Title and Id claim type constants for envelope-related information, with XML documentation for clarity.

EnvelopeGenerator.GeneratorAPI/EnvelopeClaimTypes.cs

@@ -0,0 +1,18 @@
+namespace EnvelopeGenerator.GeneratorAPI
+{
+    /// <summary>
+    /// Provides custom claim types for envelope-related information.
+    /// </summary>
+    public static class EnvelopeClaimTypes
+    {
+        /// <summary>
+        /// Claim type for the title of an envelope.
+        /// </summary>
+        public static readonly string Title = $"Envelope{nameof(Title)}";
+
+        /// <summary>
+        /// Claim type for the ID of an envelope.
+        /// </summary>
+        public static readonly string Id = $"Envelope{nameof(Id)}";
+    }
+}