WebAPI for database integrity added

Restructure the solution
* The project has been split into the database integration "src/webapi" and the Blazor wasm project "src/y4f".

WebAPI
* API with CRUD capabilities

* The main task of the "WebAPI" project is to access the database data and make it available to the Blazor-WASM project via an API.

* GET example of the allergy table via the file "TestFetchAllergienData.razor", which can be tested via the browser "/TestFetchAllergienData".

src/WebApi/Models/Kunde.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+
+namespace WebApi.Models;
+
+public partial class Kunde
+{
+    public int Idkunde { get; set; }
+
+    public string? Code { get; set; }
+
+    public int? Treuepunkte { get; set; }
+
+    public virtual ICollection<Bestellungsposition> Bestellungspositions { get; set; } = new List<Bestellungsposition>();
+}