feat(Annotation): add Annotation entity for document receiver element annotations

- Added Annotation entity mapped to table TBSIG_DOCUMENT_RECEIVER_ELEMENT_ANNOTATION
- Included data annotations for key, required fields, and relationships
- Added conditional compilation for .NET and .NET Framework compatibility

EnvelopeGenerator.Domain/Entities/Annotation.cs

@@ -0,0 +1,56 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+#if NETFRAMEWORK
+using System;
+#endif
+
+namespace EnvelopeGenerator.Domain.Entities
+#if NET
+    ;
+#elif NETFRAMEWORK
+    {
+#endif
+
+[Table("TBSIG_DOCUMENT_RECEIVER_ELEMENT_ANNOTATION")]
+public class Annotation
+{
+    [Key]
+    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+    [Column("GUID", TypeName = "int")]
+    public int Guid { get; set; }
+
+    [Required]
+    [Column("ELEMENT_ID", TypeName = "int")]
+    public int ElementId { get; set; }
+
+    [Required]
+    [Column("NAME", TypeName = "nvarchar(100)")]
+    [StringLength(100)]
+    public string Name { get; set; }
+
+    [Required]
+    [Column("VALUE", TypeName = "nvarchar(max)")]
+    public string Value { get; set; }
+
+    [Required]
+    [Column("ADDED_WHEN", TypeName = "datetime")]
+    public DateTime AddedWhen { get; set; }
+
+    [Column("CHANGED_WHEN", TypeName = "datetime")]
+    public DateTime? ChangedWhen { get; set; }
+
+    [Column("CHANGED_WHO", TypeName = "nchar(100)")]
+    [StringLength(100)]
+    public string
+#if NET
+        ?
+#endif
+        ChangedWho { get; set; }
+
+    [ForeignKey("ElementId")]
+    public virtual Signature Element { get; set; }
+}
+
+#if NETFRAMEWORK
+    }
+#endif