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/CreateEnvelopeCommand.cs

@@ -0,0 +1,24 @@
+using MediatR;
+using System.ComponentModel.DataAnnotations;
+using System.Text.Json.Serialization;
+
+namespace EnvelopeGenerator.Application.Envelopes.Commands;
+
+/// <summary>
+/// Befehl zur Erstellung eines Umschlags.
+/// </summary>
+/// <param name="Title">Der Titel des Umschlags. Dies ist ein Pflichtfeld.</param>
+/// <param name="Message">Die Nachricht, die im Umschlag enthalten sein soll. Dies ist ein Pflichtfeld.</param>
+/// <param name="TFAEnabled">Gibt an, ob die Zwei-Faktor-Authentifizierung für den Umschlag aktiviert ist. Standardmäßig false.</param>
+public record CreateEnvelopeCommand(
+    [Required] string Title,
+    [Required] string Message,
+    bool TFAEnabled = false
+    ) : IRequest<CreateEnvelopeResponse?>
+{
+    /// <summary>
+    /// Id of receiver
+    /// </summary>
+    [JsonIgnore]
+    public int? UserId { get; set; }
+};