Add EnvelopeDto and EnvelopeService for API integration

Introduced the `EnvelopeDto` class to represent envelope data with JSON property mappings. Added the `EnvelopeService` class to handle API interactions, including fetching envelopes with optional filters and query string construction using `Microsoft.AspNetCore.WebUtilities`. Updated the project file to include the required package reference for query string manipulation.

EnvelopeGenerator.ReceiverUI/Models/EnvelopeDto.cs

@@ -0,0 +1,24 @@
+using System.Text.Json.Serialization;
+
+namespace EnvelopeGenerator.ReceiverUI.Models;
+
+public class EnvelopeDto
+{
+    [JsonPropertyName("id")]
+    public int Id { get; set; }
+
+    [JsonPropertyName("uuid")]
+    public string? Uuid { get; set; }
+
+    [JsonPropertyName("title")]
+    public string? Title { get; set; }
+
+    [JsonPropertyName("status")]
+    public int Status { get; set; }
+
+    [JsonPropertyName("docResult")]
+    public byte[]? DocResult { get; set; }
+
+    [JsonPropertyName("envelopeReceivers")]
+    public List<EnvelopeReceiverDto> EnvelopeReceivers { get; set; } = new();
+}