Refactor Signature record and update EnvelopeReceiverController

- Removed default email assignment in Signature record.
- Eliminated unused using directive in EnvelopeReceiverController.
- Added new parameters for document execution and connection string options in the controller's constructor.

EnvelopeGenerator.Application/EnvelopeReceivers/Commands/Create/CreateEnvelopeReceiverDtos.cs

@@ -25,12 +25,12 @@ public record Signature([Required] double X, [Required] double Y, [Required] int
 /// <param name="PhoneNumber">Sollte mit Vorwahl geschrieben werden</param>
 public record ReceiverGetOrCreateCommand([Required] IEnumerable<Signature> Signatures, string? Salution = null, string? PhoneNumber = null)
 {
-    private string? _emailAddress = "h.tek@digitaldata.works";
+    private string? _emailAddress;
 
     /// <summary>
     /// E-Mail-Adresse des Empfängers.
     /// </summary>
-    public string? EmailAddress { get => _emailAddress?.ToLower(); init => _emailAddress = _emailAddress?.ToLower() ?? "h.tek@digitaldata.works"; }
+    public string? EmailAddress { get => _emailAddress?.ToLower(); init => _emailAddress = _emailAddress?.ToLower(); }
 };
 
 /// <summary>