feat(CreateAnnotationCommand): implement CreateAnnotationCommand and handler logic

Added full implementation for CreateAnnotationCommand and its handler:
- Introduced `PSPDFKitInstantJSON` property to the command
- Injected repositories for `Signature` and `Annotation`
- Implemented query filtering for Envelope and Receiver
- Added annotation creation from parsed PSPDFKit JSON
- Created helper method `ParsePSPDFKitInstantJSON` for JSON parsing

EnvelopeGenerator.Domain/Entities/Signature.cs

@@ -15,7 +15,7 @@ namespace EnvelopeGenerator.Domain.Entities
 #endif
 
 [Table("TBSIG_DOCUMENT_RECEIVER_ELEMENT", Schema = "dbo")]
-public class Signature : ISignature
+public class Signature : ISignature, IHasReceiver
 {
     public Signature()
     {
@@ -108,7 +108,12 @@ public class Signature : ISignature
     public virtual Document Document { get; set; }
 
     [ForeignKey("ReceiverId")]
-    public virtual Receiver Receiver { get; set; }
+    public virtual Receiver
+#if NET
+        ?
+#endif
+        Receiver
+    { get; set; }
 
     public virtual IEnumerable<Annotation>
 #if NET