Add SenderAppType enum and integrate into SignatureDto

A new `SenderAppType` enum was introduced in the
`EnvelopeGenerator.ReceiverUI.Models.Constants` namespace, with
values `LegacyFormApp` and `ReceiverUIBlazorApp`.

The `SignatureDto` class was updated to include a new property of
type `SenderAppType`, enabling the specification of the sender
application type for a signature.

Additionally, a `using` directive for the
`EnvelopeGenerator.ReceiverUI.Models.Constants` namespace was added
to `SignatureDto.cs` to support the use of the new enum.

EnvelopeGenerator.ReceiverUI/Models/Constants/SenderAppType.cs

@@ -0,0 +1,8 @@
+namespace EnvelopeGenerator.ReceiverUI.Models.Constants
+{
+    public enum SenderAppType
+    {
+        LegacyFormApp = 0,
+        ReceiverUIBlazorApp = 1
+    }
+}