Enhance envelope creation functionality and configuration

- Updated DependencyInjection to include new command handler.
- Modified CreateEnvelopeReceiverCommand for nullable responses.
- Altered SQL command in EnvelopeReceiverAddReadSQL.
- Refactored EnvelopeReceiverController with new dependencies and improved CreateAsync method.
- Expanded appsettings.json with additional configuration settings.
- Introduced new DTOs for signatures and receivers in CreateEnvelopeReceiverDtos.

EnvelopeGenerator.Application/DependencyInjection.cs

@@ -7,6 +7,7 @@ using DigitalData.Core.Client;
 using QRCoder;
 using EnvelopeGenerator.Application.Contracts.Services;
 using System.Reflection;
+using EnvelopeGenerator.Application.EnvelopeReceivers.Commands.Create;
 
 namespace EnvelopeGenerator.Application;
 
@@ -58,6 +59,7 @@ public static class DependencyInjection
         services.AddMediatR(cfg =>
         {
             cfg.RegisterServicesFromAssembly(Assembly.GetExecutingAssembly());
+            cfg.RegisterServicesFromAssembly(typeof(CreateEnvelopeReceiverCommandHandler).Assembly);
         });
 
         return services;