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/CreateEnvelopeCommandHandler.cs

@@ -0,0 +1,49 @@
+using AutoMapper;
+using EnvelopeGenerator.Application.Contracts.SQLExecutor;
+using EnvelopeGenerator.Domain.Entities;
+using MediatR;
+using Microsoft.Data.SqlClient;
+
+namespace EnvelopeGenerator.Application.Envelopes.Commands;
+
+/// <summary>
+/// 
+/// </summary>
+public class CreateEnvelopeCommandHandler : IRequestHandler<CreateEnvelopeCommand, CreateEnvelopeResponse?>
+{
+    private readonly ISQLExecutor<Envelope> _sqlExecutor;
+
+    private readonly IMapper _mapper;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="sqlExecutor"></param>
+    /// <param name="mapper"></param>
+    public CreateEnvelopeCommandHandler(ISQLExecutor<Envelope> sqlExecutor, IMapper mapper)
+    {
+        _sqlExecutor = sqlExecutor;
+        _mapper = mapper;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    public async Task<CreateEnvelopeResponse?> Handle(CreateEnvelopeCommand request, CancellationToken cancellationToken)
+    {
+        object[] parameters = new object[]
+        {
+            new SqlParameter("@UserId", request.UserId),
+            new SqlParameter("@Title", request.Title),
+            new SqlParameter("@TfaEnabled", request.TFAEnabled ? 1 : 0),
+            new SqlParameter("@Message", request.Message)
+        };
+
+        var envelope = await _sqlExecutor.Execute<CreateEnvelopeSQL>(cancellationToken, parameters).FirstOrDefaultAsync();
+
+        return _mapper.Map<CreateEnvelopeResponse>(envelope);
+    }
+}