Refactor: Replace Signatures with DocReceiverElements

Updated the codebase to replace the `Signature` record with the new `DocReceiverElementCreateDto` record for better alignment with the domain model.

- Updated `EnvelopeReceiverController` to use `DocReceiverElements` instead of `Signatures` when iterating over `Receivers`.
- Replaced the `Signature` record with `DocReceiverElementCreateDto` in `CreateEnvelopeReceiverCommand`.
- Updated `ReceiverGetOrCreateCommand` to use a `DocReceiverElements` property instead of `Signatures`.

These changes ensure consistency and reflect a shift in how document-related data is represented.

EnvelopeGenerator.API/Controllers/EnvelopeReceiverController.cs

@@ -200,7 +200,7 @@ public class EnvelopeReceiverController : ControllerBase
                 SELECT @OUT_SUCCESS as [@OUT_SUCCESS];";
 
         foreach (var rcv in res.SentReceiver)
-            foreach (var sign in request.Receivers.Where(r => r.EmailAddress == rcv.EmailAddress).FirstOrDefault()?.Signatures ?? Enumerable.Empty<Application.EnvelopeReceivers.Commands.Signature>())
+            foreach (var sign in request.Receivers.Where(r => r.EmailAddress == rcv.EmailAddress).FirstOrDefault()?.DocReceiverElements ?? Enumerable.Empty<Application.EnvelopeReceivers.Commands.DocReceiverElementCreateDto>())
             {
                 using SqlConnection conn = new(_cnnStr);
                 conn.Open();