Refactor DI extensions and add SQL executor support

Removed `DIExtensions.cs` and integrated its content into `DependencyExtensions.cs`, enhancing the dependency injection setup with repository and SQL executor registrations. Added a new `CreateEnvelopeSQL` class implementing the `ISQL<Envelope>` interface, currently with a placeholder implementation.

EnvelopeGenerator.Application/Envelopes/Commands/CreateEnvelopeSQL.cs

@@ -0,0 +1,14 @@
+using EnvelopeGenerator.Application.Contracts.SQLExecutor;
+using EnvelopeGenerator.Domain.Entities;
+
+namespace EnvelopeGenerator.Application.Envelopes.Commands;
+/// <summary>
+/// 
+/// </summary>
+public class CreateEnvelopeSQL : ISQL<Envelope>
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    public string Raw => string.Empty;
+}