Add envelope creation functionality and SQL integration

- Updated `EnvelopeGenerator.Application.csproj` to include `Microsoft.Data.SqlClient` package.
- Refactored `CreateEnvelopeReceiverCommand` to inherit from `CreateEnvelopeCommand`.
- Enhanced `CreateEnvelopeSQL` with a SQL script for envelope creation.
- Introduced `CreateEnvelopeCommand` to encapsulate envelope creation data.
- Added `CreateEnvelopeCommandHandler` to process commands and interact with the database.
- Created `CreateEnvelopeResponse` class for handling responses from envelope creation.

EnvelopeGenerator.Application/Envelopes/Commands/CreateEnvelopeResponse.cs

@@ -0,0 +1,8 @@
+namespace EnvelopeGenerator.Application.Envelopes.Commands;
+
+/// <summary>
+/// 
+/// </summary>
+public class CreateEnvelopeResponse
+{
+}