Add IDocumentExecutor interface and DocumentExecutor class

Introduces the `IDocumentExecutor` interface with a method
`CreateDocumentAsync` for asynchronous document creation.
Implements the `DocumentExecutor` class that inherits from
`SQLExecutor`, providing the functionality to create documents
using a SQL connection and handle potential errors during
the process.

EnvelopeGenerator.Application/Contracts/SQLExecutor/IDocumentExecutor.cs

@@ -0,0 +1,18 @@
+using EnvelopeGenerator.Domain.Entities;
+
+namespace EnvelopeGenerator.Application.Contracts.SQLExecutor;
+
+/// <summary>
+/// 
+/// </summary>
+public interface IDocumentExecutor
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="base64"></param>
+    /// <param name="envelope_uuid"></param>
+    /// <param name="cancellation"></param>
+    /// <returns></returns>
+    Task<EnvelopeDocument> CreateDocumentAsync(string base64, string envelope_uuid, CancellationToken cancellation = default);
+}