Refactor DI for Envelope services and add new executor

Updated DIExtensions to register IEnvelopeExecutor as a singleton and added IEnvelopeReceiverExecutor as a new singleton service. Introduced IEnvelopeReceiverExecutor interface and implemented it in the new EnvelopeReceiverExecutor class for future envelope processing functionality.

EnvelopeGenerator.Infrastructure/Executor/EnvelopeReceiverExecutor.cs

@@ -0,0 +1,7 @@
+using EnvelopeGenerator.Application.Contracts.SQLExecutor;
+
+namespace EnvelopeGenerator.Infrastructure.Executor;
+
+public class EnvelopeReceiverExecutor: IEnvelopeReceiverExecutor
+{
+}